60
J. Phys. Chem. 1982, 86, 60-66
Multlphoton Ionization-Fragmentation Patterns of Alkyl Iodides
D. H. Parker and R. B. Bernsteln'
Department of chemistry, Columbia University, New York, New York 10027 (Received: September 1, 1981)
Multiphoton ionization (MP1)-fragmentation patterns are reported for six alkyl iodides (methyl,ethyl, n-propyl,
isopropyl, n-butyl, and tert-butyl) in the 4W360-nm region. Three- or four-photon ionization is resonantly
enhanced via two-photon excitations to various spin-orbit components of the 5pa 6s molecular Rydberg
transition. For each of these molecules a minimum of seven photons (overall) must be absorbed to account
'). For methyl iodide it appears that different
for the observed ion fragments (which include C+ and H
intermediate Rydberg states can affect the rate of ionization and thus the subsequent ion fragmentation process,
possibly due to retention by the ion of the differing Rydberg-state geometries. Such distinct fragmentation
effects are not observed for different resonant states in the larger alkyl iodides. Although the resonant-state
spectra are essentially continuous, significant structure-sensitiveinformation can be obtained by means of the
MPI mass spectral (MPIMS) technique. For the isomers n-propyl iodide and isopropyl iodide (whose electron
impact mass spectra are essentially identical) the MPIMS patterns are significantly different; similarly for
tert-butyl iodide and n-butyl iodide. This is attributed to differences in the spectroscopic and dynamics properties
of the first-formed fragment ion in the MPI-fragmentation process. Specifically, the n-propyl and n-butyl
cations do not isomerize appreciably within the 5-ns laser pulse duration but absorb the 368-nm radiation and
photodissociate, whereas the isopropyl and tert-butyl cations are essentially transparent at this wavelength
and thereby persist without fragmentation.
-
Introduction
Much progress has been made over the past several years
in understanding resonance-enhanced multiphoton ionization-dissociation (REMPID) processes in polyatomic
molecules. REMPI is, in itself, a mature branch of twoand three-photon spectroscopy' in which the intensity of
a (typically) three- or four-photon ionization process is
strongly enhanced by the existence of an intermediate
excited electronic state, often a molecular Rydberg state,'
reached via two- or three-photon absorption. Analysis of
the usually extensive ion photofragmentation which takes
place during the laser pulse duration has been the subject
of much recent research.2 One motivation for such study
is in the development of REMPI as a tool for analytical
mass ~pectrometry.~
Several techniques have been used to examine the successive steps in the REMPID process. Kinetic-energy
analysis4 of the photoelectrons, cations, and anions5 has
established for several molecules that fragmentation takes
place up the "ionic ladder" (rather than via the neutral
(1)(a) P. M. Johnson, Acc. Chem. Res., 13, 20 (1980); (b) P. M.
Johneon, Annu. Reu. Phys. Chem., in press; (C) D. H. Parker, J. 0. Berg,
and M. A. El-Sayed in "Advancea in Laser Chemistry", A. H. Zewail, Ed.,
Springer, West Berlin, 1978, p 320.
(2)(a) U.Boesl, H. J. Neueser, and E. W. Schlag, Z . Naturjorsch. A,
33, 1546 (1978); (b) L. Zandee, R. B. Bernstein, and D. A. Lichtin, J.
Chem. Phys., 69, 3427 (1978;L. Zandee and R. B. Bernstein, ibid., 70,
2574 (1979);71, 1359 (1979); (c) V. S. Antonov, I. N. Knyazev, V. S.
Letokhov, V. M. Matiuk, V. G. Movshev, and V. K. Potapov, Opt. Lett.,
3, 37 (1978); (d) S. D. Rockwood, J. P. Reiily, K. Hohla, and K. L.
Kompa, Opt. Commun., 28,175(1979);J. P.Reilly and K. L. Kompa in
'Laser Spedroscopy", VoL IV,Springer-Verlag,West Berlin, 1979;p 631;
(e) D. A. Lichtin, S. Datta-Ghoeh, K. R. Newton, and R. B. Bemstein,
Chem. Phys. Lett., 75,214 (1980);K. R. Newton, D. A. Lichtin, and R.
B. Bemstein, J. Phys. Chem., 85, 15 (1981); (f) T.G. Dietz, M. A.
Duncan, M. G. Liverman, and R. E. Smalley, J. Chem. Phys., 73, 4816
(1980).
(3)D. A. Lichtin, L. Zandee, and R. B. Bemstein, Chap. 6 in "Lasers
in Chemical Andy&",G. Hieftje, J. Travis, and F. Lytle, Eds., Humana
Press, Clifton, NJ, 1981,p 125.
(4)(a) J. T.Meek, R. K. Jones, and J. P. Reilly, J.Chem. Phys., 73,
3503 (1980); (b) J. C. Miller and R. N. Compton, ibid., 75, 22 (1981).
(5)M. S. DeVries, N. J. A. Van Veen, T. Baller, and A. E. DeVries,
Chem. Phys., 56, 157 (1981).
0022-3654/82/2086-0060$0 1.2510
autoionization route). Two-laser studies6 have confirmed
this for several cases and have also indicated the importance of the parent-ion absorption spectrum in determining
the extent and the laser-power dependence of fragmentation. Much more detailed information on this aspect is
provided by a combined electron impact-laser technique'
where (parent and daughter) ions are formed by electron
impact and then selectively dissociated by a pulsed tunable
dye laser.
A number of other studies, experimental"" and theoretical,12J3have contributed to the understanding of the
REMPID process. In many cases it appears that the total
energy available to the parent ion after multiphoton ionization occurs is important,'OJ1 but the manner in which
the ion was formed is not. This premise is tested for a
number of simple alkyl iodides; evidence to the contrary
will be presented for methyl iodide.
The usefulness of REMPID to analytical mass spectroscopy will be governed by sensitivity and selectivity.
Although the duty factor of present pulsed lasers is low
(cf. CW electron impact sources),the ionization probability
per pulse is typically greater for photons. Spectroscopic
selectivity is the major advantage of MPI, however. Selective photoionization of isomeric and isotopic molecules
has been demonstrated.'&"j For the isomers azulene and
(6)(a) U.Boesl, H. J. N e w e r , and E. W. Schlag, J. Chem. Phys., 72,
4327 (1980); (b) K.R. Newton, D. A. Lichtin, and R. B. Bernstein, J.
Phys. Chem., 85, 15 (1981).
(7)K. R. Newton and R. B. Bematein, to be submitted for publication.
(8)G. J. Fisanick, T. S.Eichelberger, B. A. Heath, and M. B. Robin,
J. Chem. Phys., 72,5571 (1980).
(9)R. Pandolfi, D. A. Gobell, and M. A. El-Sayed, J. Phys. Chem., 85,
1779 (1981).
(10)T.Carney and T. Baer, J. Chem. Phys., 75,477 (1981).
(11)D. H. Parker, D. A. Lichtin, and R. B. Bemstein, J. Chem. Phys.,
75,2577 (1981).
(12)J. Silberstein and R. D. Levine, Chem. Phys. Lett., 74,6 (1980).
(13)(a) F. Rebentrost and A. Ben-Shaul, J. Chem. Phys., 74,3255
(1981); (b) F. Rebentrost, K. L. Kompa, and A. Ben-Shad, Chem. Phys.
Lett., 77,394 (1981).
(14)D. M. Lubman, R. Naaman, and R. N. Zare, J. Chem. Phys., 72,
3034 (1980).
0 1982 American Chemical Society
MPI-Fragmentation Patterns of Alkyl Iodides
The Journal of Physical Chemistry, Vol. 86, No. 1, 1982 61
naphthalene the laser can be wavelength-tuned to a resonance of one molecule, ionizing it without exciting the
other isomer.14 Similar results have been reported for
substituted anilines.’& In both studies the fragmentation
patterns, both E1 and MPI, of the two isomers were readily
distinguishable. The geometric isomers cis- and transdichloroethylene can be selectively ionized by appropriately tuning the laser.l6 Here the fragmentation patterns,
both E1 and MPI, were found to be essentially identical.
In the present study it will be shown that the isomers
isopropyl iodide and n-propyl iodide, whose E1 fragmentation patterns are almost indistinguishable, cannot be
wavelength-selected because of overlapping absorption
spectra; yet they can be easily distinguished by their very
different REMPID patterns. Similar results are also noted
for the isomers tert-butyl iodide and n-butyl iodide.
As in the previously reported MPI-fragmentation study
of the tertiary amines,” considerable structure-sensitive
information can be obtained from the REMPID results
despite the absence of resolvable spectroscopic features
in the REMPI bands. Thus, many molecules formerly
believed to be less interesting for study by MPI spectroscopy now become important subjects of investigation
via laser mass spectrometry. These include the alkyl iodide
family, six members of which have been chosen for the
present study.
Experimental Section
The laser ionization time-of-flight mass spectrometer
(TOFMS) has been described in detail previously.2BLaser
wavelengths useful in this study were generated by mixing
the fundamental of a Nd:YAG laser with the output of a
tunable dye laser, pumped by the NdYAG second harmonic. A 250-mm fl lens was used to focus the final laser
beam into the ionization region of the TOFMS. Laser
pulse energies (mJ) were measured by a calorimeter.
Electron impact mass spectra a t 70 eV were recorded for
comparison with MPI-fragmentation patterns obtained
in the same apparatus. Sample pressures were typically
1 x iO-5-2 x 10“ torr.
The samples used were commercial grade, degassed
before use. The larger alkyl iodides were found to contain
small fractions of isomeric impurities as well as some
methyl iodide. No effort was made to correct for these,
except that laser wavelengths were chosen to avoid methyl
iodide absorptions. The integrated ion signal vs. wavelength (i.e., the MPI spectrum) for each molecule was
found to be in good agreement with known MPI cell
spectral7 (except for isopropyl iodide and tert-butyl iodide,
whose spectra have not been published). In the next
section TOFMS results for all six alkyl iodides will be
summarized and discussed.
Results and Discussion
REMPID patterns as a function of laser pulse energy
will be presented for methyl, ethyl, n-propyl, isopropyl,
t
n-butyl, and tert-butyl iodides and compared with E1
spectra at 70 eV. Additional MPI mass spectra at different
wavelengths will be displayed for methyl iodide. The excitation wavelengths were chosen to coincide with the
vibrationless origins of the various spin-orbit components
(15)(a) A. Herrmann, S. Leutwyler, E. Schumacher, and L. Wbete,
Chem. Phys.Lett., 52,418 (1977);(b) E.W.Rothe, B. P. Mathur, and
G. P. Reck, ibid., 53,74(1978);(c) C.T.Rettner and J. H. Brophy, Chem.
Phys.,56,53 (1981).
(16)J. W.Hudgens, M. Seaver, and J. J. DeCorpo,J. Phys.Chem., 86,
761 (1981).
(17)D. H.Parker, R. Pandolfi, P. R. Stannnrd, and M. A. El-Sayed,
Chem. Phys.,45, 27 (1980).
nethyl iodide
CH31
d P I 3 7 0 n m (A1 v.0)
3.0mj
CH?
I
I+
1
2.4mj
I
0.4mj
20
0
120
MASS
140
Figwe 1. REWID mass spectra of methyl iodide, two-photon resonant
with the %, A, origin, at several specified laser pulse energies (mJ
pulse-‘). Bottom panel shows E1 mass spectrum at 70 eV for comparison.
methyl iodide
CH3I
M P I 3 7 0 n m 1.6mj
I+
H+
P
CHf
C+
hh
‘
5
L
15
130
140
MASS
Flguro 2. Higher-resoiution REMPID mass spectrum of methyl iodide
extended down to H+.
-
of the 5 p r
6s molecular Rydberg transition.l8 Of
particular interest are the Al and E components (3?r0 and
‘r) lying near 370 and 366 nm for each m01ecule.l~.No
significant differences in the total ion currents were observed between isopropyl iodide and n-propyl iodide or
(18)(a) G.Herzberg, “Molecular Spectra and Molecular Structure”,
Vol. 111, Van Nostrand, Princeton, NJ, 1967. (b) J. D. Scott, W. S. Felps,
G. L. Findley, and S. P. McGlynn, J. Chem. Phys.,68, 4678 (1978).
82
The Journal of Physical Chemistty, Vol. 86,No. 1, 1982
24
20
Parker and Bernstein
It
-
!
~
CH,I
I
0 '
I
Figure 9. Energy-level dlagram for REMPID of methyl lcdkle showlng
two-photon resonant three-photon ionization followed by successive
photofragmentation of CH31+ (level 3). Dashed lines are estlmated;
solid Hnes are known appearance potentials of (possiMe) ion fragments.
between tert-butyl iodide and n-butyl iodide.
Methyl Iodide. Figure 1shows the REMPID patterns
for methyl iodide at several laser pulse energies, for a laser
wavelength of 370 nm, resonant with the Al origin by
two-photon absorption. To be noted is the typically observed continuous shiftii of fragmentation toward smaller
masses with increasing laser pulse energy. The 70-eV E1
spectrum of methyl iodide is shown in the bottom panel
of Figure 1. The total ion current was found to increase
as the square of the laser pulse energy, consistent with a
two-photon resonant-state "bottleneck".
Higher-resolution scans of the methyl iodide fragmentation pattern are shown in Figure 2 for low- and high-maas
regions. These spectra were taken in separate scans in
order to include H+, which has not been monitored in the
other REMPID spectra in this paper. (The apparent
relative intensity of H+ was found to vary from time to
time much more than that for the larger ions.) Although
the similarity of the CH,+ and CH,I+ patterns (n = 0, 1,
2,3) is striking in this figure, such similarity was not always
observed at other laser wavelengths and pulse energies.
The low relative abundance of CH+ and CHI+ is general
for REMPID of methyl iodide. However, this is not found
in the E1 spectrum.
The qualitiative features of REMPID of methyl iodide
can be interpreted with the aid of Figure 3. Shown are
the appearance potentials of the possible ionic fragments
of methyl iodide vs. the minimum number of near-UV
photons. Level 1 is a virtual state in the two-photon resonant three- or four-photon ionization process. Two
ionization potentials are shown for methyl iodide (level 3)
corresponding to the spin-orbit components 2E3/2 and
?E+!. The REMPID spectra shown in Figures 1and 2 are
resonance enhanced via the AI (3?ro) Rydberg state which
correlates with the upper (2El/2)ionization potential.
Solid lines in Figure 3 indicate measuredlg appearance
potentials; those with dashed lines are estimated. Disso(19) H.M. Roaenstock, K.Draxl, D. W. Steiner, and J. T. Herron, J.
Phys. Chem. Ref. Data, 6, Suppl. No. 1 (1977).
INTENSITY
IO
PHOTON ENERGY (cm-')
Flgure 4. Detalled energy-level diagram for REMPI of methyl iodide
via several two-photon resonant states. Threaphoton ionization is
posdble for photon energies above 27 250 cm-'. The lower 3 ~ E1state
correlates with the *Ejl2 CH3I' otentlal; the upper 37r0A, and 'iTl E
states correlate with E,,, CH31 . Also shown are the photoelectron
spectrum and the UV absorption spectrum of CHJ including the A,
( v = 0, 1) bands, marked by asterisks, which appear only in the
two-photon spectrum.
P
ciation to CH3+ can produce two atomic iodine components, U2P32) and I*(2P1/2).The lower appearance pois based upon an empirically adjusted heat
tential for
of formation.20 From this diagram it is evident that at
least seven photons are required to produce C', H+, and
CH+ from CH31. It is possible that even more photons are
actually absorbed since the intermediate dissociation
species are known4to possess kinetic and internal excess
energy.
More information on the detailed photophysics of
methyl iodide REMPID can be obtained by varying the
laser wavelength. Three separate electronic transitions are
accessible by two-photon resonant MPI in this region,
corresponding to the 3 ~ E1 state and the 3 ~ Al
0 and lr1E
excited states from the 5p7r
6s molecular Rydberg
transition. The energy positioning of these states and other
relevant information for methyl iodide REMPID is shown
in Figure 4. The Al state, which is not observed by conventional one-photon absorption spectroscopy, is included
in the UV absorption spectrum. Asterisks mark its vibrationless origin and the first quantum of the main vibrational progression-the Yg totally symmetric methyl
umbrella stretch.l* The lowest-energy excited state, the
broad absorption from 4 to 6 eV, is responsible for the
well-studied methyl iodide photochemistry.21
The photoelectron spectrum22of methyl iodide also included in Figure 4 shows the two spin-orbit components
d+
-
(20) J. Silberstein and R. D. Levine, J.Chem. Phys., 75,5735 (1981).
(21)(a) M.Dzvonik, S.Yang, and R. Bersohn, J. Chem. Phys., 61,4408
(1974); (b) M.Shapiro and R. Bersohn, ibid., 73, 3810 (1980).
(22) J . H. D. Eland, R. Frey, A. Kueetler, H. Schulte, and B. Brehm,
Int. J. Mass Spectrom. Ion Phys., 22, 155 (1976).
MPI-Fragmentatlon Patterns of Alkyl Iodides
The Journal of phvsical Chemistty, Vol. 86, No. I, 1982 63
methyl iodide C H S I
M P I 2.4mj
0
20
120
140
MASS
Flgure 5. REMPID mass spectra for methyl Iodide at constant laser
pulse energy via separate (Indicated) Intermediate states.
of ground-state CH81+and the first excited state lying near
12 eV. The photodissociation spectrum of CH31+,which
has also been reportedB for the lower region of the first
excited state, is similar (while resolving some vibrational
fine structure).
The lowest observed state, E ( 3 ~ 1 )with vibrationless
origin at 6.12 eV, correlates in the limit of strong spin-orbit
coupling with the E3/2 component of CH31+while the other
two components, Al and E
and ~ T J correlate
,
with the
ElIz component. For this upper level four laser photons
(A 1 367 nm) are required for ionization. The vibrationless
origin of the second E state and the 6; band of the Al state
lie in the three-photon ionization region. MPI cell spectral7
show only a slow rise in ionization signal when crossing this
limit.
Figure 5 shows the REMPID patterns of methyl iodide
a t constant laser power for the four resonances shown in
Figure 4. The longest-wavelength excitation (402 nm)
results in the most extensive fragmentation observed-as
indicated by the almost complete loss of parent ion. The
total ion yield is low because of the four-photon requirement, however, and is consistent with the relative intensities of the REMPI cell spectra.17 Such fragmentation
behavior can be easily understood from Figure 4, where
the fourth photon is seen to lie in the region of the first
excited states of CHSI+. The “doorway” to further absorption and resulting fragmentation is thus “open” compared to the shorter wavelengths where the fourth photon
terminates in a region of weak ion absorption. This
(23)D.C. McGilvery and J. D. Morrison, J. Chem. Phys., 67, 368
(1977).
mechanism has recently been shown to control the extent
of fragmentation in other molecules also, notably hexadiyne’O and the tertiary amines.’,
Further information can be obtained from the three
higher-energy resonances. The results shown in Figure 5
indicate a much larger degree of fragmentation for the (u
= 0 and 1) Al bands than for the interposed E(v = 0) band.
(To obtain the same degree of fragmentation, it is necessary to double the laser pulse energy when resonant with
the E state?, The total ion current is again consistent with
the REMPI cell spectra where the relative ion yield is
roughly 2:1:0.5 for the A,(u = 0), E(u = 0), and A,(u = 1)
bands, respectively. Since the ion absorption spectrum is
essentially continous in this
it is difficult to rationalize this fragmentation trend through a chance
doorway variation. A correlation with the type of intermediate resonant state selected (A, vs. E) seems indicated
by the marked similarity of the two A, band fragmentation
patterns.
A qualitative explanation for this possible dependence
on the resonant intermediate state arises from consideration of the excited-state geometry. Rotational line-shape
analysis17of the REMPI spectra suggested that the HCH
bond angle is larger for the A, state than for the ground
state, while the E-state and ground-state geometries are
very similar. An approach toward planarity of the methyl
group would elongate the C-I bond, making the molecule
less stable. As noted, REMPID through the A, levels
results in significantly more fragmentation (at a given laser
pulse energy) that the E level. If the different geometries
of the two Rydberg states are retained in the ion, different
fragmentation could be expected. Successive photoabsorption must take place very quickly compared to collisional relaxation effects-with a long-time limit of the
-5-ns laser pulse length.
Similar effects were not detected in the REMPID patterns of the larger alkyl iodides. Becuase of the almost
diffuse spectra of these systems, caused in part by sequence
congestion, it is much more difficult to excite a single
resonant state component.
REMPZD Patterns for Larger Alkyl Zodides. Total ion
currents were found to increase quadratically with laser
pulse energies, as expected, for the four larger RI molecules. REMPID patterns for ethyl iodide at several laser
pulse energies are compared with the 70-eV E1 mass
spectrum in Figure 6. The laser wavelength at 372 nm
is two-photon resonant with the Al absorption region.
Similar fragmentation patterns were observed with other
wavelengths at similar laser powers. Almost no parent ion
or parent minus hydrogen(s) is observed with MPI, even
for the lowest laser pulse energies, indicating strong parent-ion absorption at these wavelengths. The appearance
of C+ and Cz+at higher pulse energy is also noted in Figure
6. The observed changes in the fragmentation patterns
for a fourfold increase in laser power are rather small and
will be discussed later. However, since the “expensive”
C+ ions are formed, a large number of photons must be
absorbed at high laser powers.
Another interesting difference between the REMPID
and E1 mass spectra concerns mass 28 (CZH4’). As shown
in more detail in Figure 7, almost no CzH4+is observed
with MPI, in contrast to EI. REMPID of molecules with
similar alkyl groups (e.g., triethylamine) at similar laser
wavelengths has been shown to produce an abundance of
C2H4+.I1 This ion is simply not formed in the REMPID
of ethyl iodide.
The REMPID and E1 patterns for the isomers n-propyl
iodide and isopropyl iodide are compared in Figure 8. The
84
The Journal of Physical Chemistry, Vol. 86, No. 1, 7982
Parker and Bernstein
observed
in the tertiary amines." A major difference,
m2&Y---l
however,
is that here a large number of fragments are
C2H i+
M P I 372nm
0.5mj
20
120
MASS
140
160
Flgure 8. REMPID mass spectra for ethyl Iodide at different lndlcated
laser pulse energies, compared wth E1 at 70 eV.
e l h y l iodide
QH5I
MPI 365 nm 0 . 4 mi
b
€ 1 70cV
-
7-
25
30
MASS
Figure 7. Higher-resoMionREMPID pattems for ethyl kdide compared
with E1 mass spectrum. Note the near absence of mass 28 (C2H,+)
via REMPID.
strong similarity of the E1 spectra, taken under identical
conditions, precludes distinguishing these isomers by their
E1 spectra. However, their REMPID patterns are very
different at the same wavelengths and pulse energy.
For both isomers the parent ions are *burned out" by
the laser photons of 368 nm. But consider the REMPID
patterns for isopropyl iodide. No significant change in
fragmentation is observed over a threefold range of laser
pulse energy. The total ion yield is found to increase
strongly with laser power, as expected. Such behavior,
although atypical of most REMPID patterns, has also been
observed and found independent of laser power in contrast
to only two species (the parent and a large daughter ion)
for the tertiary amines. It is to be expected that at least
some of the fragments would further absorb photons from
the laser pulse, leading to increasing fragmentation with
increasing laser power.
Similar effects are observed for n-propyl iodide, though
less pronounced. For a fourfold increase in pulse energy,
only a small fraction of fragments (C+,C2+,and CH+ ions)
are generated. This is again atypical REMPID behavior.
Only the I+ yield shows a dependence on the laser power,
for both propyl iodide and butyl iodide.
n-Butyl iodide and tert-butyl iodide are compared in
Figure 9 for REMPID and 70-eV EI. The two molecules
can be distinguished by EI, with the tert-butyl isomer
fragmenting to a larger extent. The appearance of CzH4+
should also be noted in both E1 spectra. The REMPID
patterns are most surprising for tert-butyl iodide. Essentially only one ion is observed (a fragmentation) over
a threefold energy range. In contrast, the REMPID patterns for n-butyl iodide show extensive fragmentation (no
parent ion present) which, once more, has only a slight
dependence on the laser power. tert-Butyl iodide fragments mainly to the (CHd C+ ion, with almost no parent
ion formation. (Asfor ethy! iodide and the propyl iodides,
again no C2H4+is observed in the REMPID mass spectra.)
For tert-butyl iodide, the lack of fragmentation with increasing laser pulse energy implies that the tert-butyl
cation (or its precursor) does not dissociatively absorb at
this laaer wavelength. Either this cation is formed directly
via the absorption of a fourth photon or a precursor such
as a highly excited parent ion produces the tert-butyl ion
in a short time (ca.1 ps) after absorbing the fourth photon.
The latter suggestion is attractive for interpretation of
REMPID of the other alkyl iodides.
If the precursor of the observed fragments has a lifetime
7 2 T ~the
,
laser pulselength, most of these fragments will
not be excited (and further dissociated) by the laser. The
complicated fragmentation pattern will be unchanged with
increasing pulse energies (although the overall ion yield
increases). Less-stable precursors will yield fragments
earlier (7< 71) and thus show stronger fragmentation-pulse
energy dependence.
Metastable ions, commonly found in E1 studies, have
been recently observed also in MPI mass spectra.24" Only
when the precursor lifetime is of the order of the ion acceleration time (Le., microseconds) can the metastable
processes be detected (via peak broadening). Metastable
lifetimes 7 = T~ (-5 ns) are consistent with this mechanism,
which accounts for most of the general features of the
REMPID spectra. For experiments at increasing laser
power, multiphoton absorption by the precursor should
eventually dominate, yielding more complex fragmentation
behavior. Further experiments are obviously necessary to
gain a better understanding of the REMPID process.
Concluding Remarks
For methyl iodide several resonant states have been
compared and found to affect the subsequent ion fragmentation process. For all of the alkyl iodide molecules,
the importance of the parent-ion absorption spectrum has
been noted. The isomer pairs n-propyl iodide-isopropyl
iodide and n-butyl iodide-tert-butyl iodide have been
(24) D. Proch, D. M. Rider, and R. N. Zare, Chem. Phys. Lett., 81,430
(1981).
(25) Unpublished results on tertiary amines from this laboratory.
MPI-Fragmentation Patterns of Alkyl
Iodides
The Journal of Physlcal Chemlsrry, Vol. 86, No. 1, 1982 65
--IF
n-propyl iodide
I
n-C3H7I
M P I 368 n m
n,-C3H7+
CzHf
I
isDpropyliodide;,CzH I
'
d
1.8mj
1.0 mi
0.5 m j
P
1
20
120
40
140
160
180
-20
120
40
140
160
180
MASS
MASS
Figure 8. REMPID and E1 mass spectra of n-propyl iodide and isopropyl iodide. Presentation similar to Figure 6.
n - b u t y l iodide
n-CqHgI
1-butyl iodide (CH+,CI
MPI
CZH2
CH?
1)
I 1,
368nm
1.5mj
M P I 368 nm
1.5mj
(CHs),C'
n- C4H9*
1
I+
I+
'1
p
, I
'
I
1.0 mj
1.0 m i
L
- , ,
0.5 mi
0.5 mi
I
20
40
60
120
MASS
140
160
180
20
.I
40
-
,
60
I
I
I
120
140
I60
180
MASS
Figure 0. REMPID and E1 mass spectra of n-butyl iodide and tert-butyl iodide. Presentation similar to Figures 6 and 8.
examined by REMPID and EI. Even in the absence of
wavelength selectivity, REMPID mass spectra contain
significant structure-sensitive information on these molecules. A simple mechanism involving metastable precur-
J. Phys. Chem. 1082, 86,66-70
66
sors has been proposed to account for the atypical behavior
of the REMPID fragmentation patterns of the alkyl iodide
molecules with increasing laser pulse energy. A key finding
of the present study is that the n-propyl and n-butyl
cations do not isomerize appreciably (to isopropyl or
tert-butyl cations, respectively) within the 5-ns laser puke
duration but absorb the 368-nm radiation and photodis-
sociate, whereas the isopropyl and tert-butyl cations are
essentially transparent at this wavelength and thereby
persist without fragmentation.
Acknowledgment. This work has been supported by
NSF Grants CHE 78-25187 and CHE 77-11384, hereby
acknowledged with thanks.
Klnetics of the Reactions of Methoxy and Ethoxy Radlcals wlth Oxygen
D. Outman,'' N. Sanders,? and J. E. Butler
Chemistry Divhbn, Nevel Research Laboratory. Washington, D.C. 20375 (Received: September 4, 1981)
Rate constants have been obtained as a function of temperature for the gas-phase reactions of methoxy and
ethoxy radicals with oxygen. The alkoxy radicals were produced by the 266-nm photolysis of the corresponding
nitrites, and their concentrationswere monitored by using laser-induced fluorescence. The CHBO+ O2reaction
was studied from 140 to 355 "C, and the rate constants obtained were used to derive an Arrhenius expression,
6.3 X lo7exp(4.6 kcal/RT) M-' s-l. The C2HS0+ O2reaction was studied at two temperatures, and the rate
constants obtained were 4.8 x lo6 M-' s-l (23 "C) and 5.9 x lo6 M-l s-l (80 "C). Upper limits for the rate of
the isomerization reaction, CH30 + M CHzOH + M,are calculated, and the possible use of the results of
this study in atmospheric smog modeling is discussed.
-
Introduction
Alkoxy radicals are important reaction intermediates
formed during the gas-phase oxidation of most hydrocarbons. In the early, low-temperature stages of combustion these labile intermediates form largely through the
decomposition of peroxidic species:
ROOR' -* RO. + R'O.
(1)
Subsequent reactions of these radicals ultimately produce
such products as aldehydes, ketones, and alcohol^.^,^ In
photochemical smog cycles, alkoxy radicals are produced
from the oxidation of nitric oxide by alkylperoxy radicals:
ROz
+ NO
--+
RO. + NOz.
(2)
Under atmospheric conditions RO- can react with oxygen
RCH20.
+02
---*
RCHO
+ H02
(3)
isomerize or d e c ~ m p o s e . ~
Although the reactions of alkoxy radicals have been the
subject of numerous studies, our present knowledge of the
reactivity of these intermediates is still largely based on
indirect kinetic measurements. The primary obstacle to
performing direct studies has been the lack of a suitably
sensitive method for dynamically monitoring these free
radicals under well-defined reaction conditions. Recently
(1) On sabbatical from the Department of Chemistry, Illinois Institute
of Technology, Chicago, IL 60616.
(2) NRL/NRC Postdoctoral Research Associate (1979-1981). Present
address: E u o n Research and Engineering Co, Linden, NJ 07036.
(3) Pollard, R. T. In "Comprehensive Chemical Kinetics";Bamford,
C. H., Tipper, C. F. H., Eds.; Elsevier: New York, 1977; Vol. 17, Chapter
2.
(4) Bradley, J. N. "Flame and Combustion Phenomena"; Methuen:
London, 1969.
(5) Pitts, J. N., Jr.; Finlayson, B. J. Angew. Chem., Int. Ed. Engl. 1975,
14. 1 and references therein.
Inoue, Akimoto, and Okuda have observed and analyzed
the laser-induced fluorescence (LIF) spectra of three simple alkoxy
These studies have now provided
the spectroscopic information needed to use LIF as a diagnostic method in future chemical kinetic studies of the
reactions of the three radicals, CH30, C2Hs0,and C2H30.
The first direct study of an alkoxy radical reaction was
reported by Sanders, Butler, Pasternack, and M~Donald.~
The investigation involved the production of CH30 using
pulsed UV laser photolysis of CH30N0 and the subsequent monitoring of the decay of CH30 by LIF in the
presence of different reactant gases. A rate constant for
the reaction with NO was obtained,1° as were upper limits
on the rate constants of nine other reactions, all at room
temperature. We wish to report that this same experimental procedure has now been used with a heated reaction vessel to study the reactions of CH30and C2H50with
oxygen at several temperatures.
Prior studies of RO + O2reactions have all been of the
simplest of the series, namely
CH30 + O2 CHzO + HOz
(4)
Three recent publications have focused on reaction 4, reporting rate constants at various temperatures."-13 All
-
(6) Inoue, G.; Akimoto, H.; Okuda, M. J. Chem. Phys. 1980,72,1769.
(7) Inoue, G.; Akimoto, H. J . Chem. Phys. 1980, 74,425.
(8)Inoue, G.; Okudo, M.; Akimoto, H. J . Chem. Phys. 1981,75,2060.
(9) Sanders, N.; Butler, J. E.; Paaternack, L. R.; McDonald Chem.
Phys. 1980,48, 203.
(IO) The room-temperaturevalue for the CH90 + NO rate constant
reported in ref 9 is valid only for the 15 i 5 torr of SF, buffer gaa pressure
used in the experiments on this reaction. Subsequent work at other
pressures confirms that this reaction is in the falloff region at 15 torr.
N.D.S. thanks Dr. L. S. Batt of Aberdeen University for pointing out the
likelihood that the mechanism of the CHsO + NO reaction under the
conditions of the earlier study waa primarily the pressure-dependent
recombiantion process.
(11) Barker, J. R.; Benson, S. W.; Golden, D. M. Int. J. Chem. Kinet.
1977, 9, 31.
(12) Batt, L.; Robinson, G . N. Int. J . Chem. Kinet. 1979, 9, 1045.
This article not subject to U.S. Copyright. Published 1982 by the American Chemical
Society