Selection of Raman Laser Materials
GISELA ECKHARDT
Abstract-In order to select Raman laser materials fora specific by the excitation of SRS extends from 3231 in the near
purpose, it is important to know which frequency shifts and excita- ultraviolet to 16600
in the near infrared [1]-[16]. More
tion power thresholds can be expected from various substances.
than
100
coherent
lines
have been observed to date from
The thresholdsare strongly dependent on the peak scattering cross
over 30 substances. This represents a considerable exsections of the corresponding incoherent Raman lines, and these
tension of the frequencyrangebeyond the shift to be
cross sections can vary by several orders of magnitude between
Raman lines from diierent molecules as well as between Raman expected from the highest fundamental vibrational frelines &om a single molecule. This paper points out how the rules
quency of any molecule, i.e., 4160 cm-' exhibited by H,.
concerning frequencies, intensities, degrees of depolarization, and
This
extension was possible because of 1) the occurrence
l i e widths, established for incoherent Raman scattering, can be
used as a guide for theprediction of Raman laser performance for of an iteration process 111, [2] (the Raman-shifted light
new materials. Subsequently,all substances andparticipating vibra- acts as exciting light), 2) the occurrence of parametric
tions which have been reported to exhibit stimulated Raman scattereffects [5], [7], and 3) the use of frequency doubling
ing to date are listed and discussed.
techniques [16], [17].
So far it has been found impossible to produce coherently
the complete Raman spectrum of any substance.
I. INTRODUCTION
Rather, only some of the most intense lines of the normal
HREE YEARShavepassed
now since the dis- Raman spectrum of a chosen substance appeared in the
covery [l] and explanation [2] of stimulated Raman SRS spectrum. An increase in the power density of the
scattering(SRS).The
effect hasproven to be a exciting laser light only enhanced the number of multivery effective method for the coverage of a large spectral ples and combinations of a few of the strongest Raman
range with intense coherent lines, produced by convert- lines [2], [5], [lo].
ing a sizable portion of the output of a &-switched laser
According to the theory of SRS by Hellwarth [ H I , [19],
(up to 30 percent) into laser light of different frequencies. the peak intensity of a Raman line is a measure .of the
Most of the papers on SRS have dealt with the many threshold height for Raman laser action.' I n addition,
interesting properties of the effectitself.However, SRS the degree of depolarization of the Raman-scattered light
also holds promise as a research tool for special spectro- is important, since this quantity enters into the calculascopic investigations; to name just a fewexamples, the tion of the cross section for polarized scattering in the
study of fast chemicalprocesses of various kinds (de- forward direction. (For a surveyonSRS
theory, see
tection of intermediate reaction products), light scattering Bloembergen [23].) Many investigators of Raman line
experiments in the atmosphere, and the study of other intensities have measuredthe integrated intensities rather
nonlinear effects which may require the use of an intense than the maximum differential intensities. Since the halflight source either of one specific frequency or containing widths for vibrational lines vary between about 2 cm-'
several specific frequencies simultaneously. Many po- and at least 40 em-', it is necessary in these cases to
tential users of SRS in applications of this type might know also the line width in order t o calculate the relafind it cumbersome to utilize the coherent Raman effect, tive threshold for SRS of a particular line. Thereare
however,because they are not especially familiar with only very limited data available on line widths and on
Raman spectroscopy. The present paper was written for line intensities relative to an intensity standard, and the
the benefit of this group of readers; it is intended to assist published information on the depolarization of Raman
in choosing new Raman laser materials if an experiment lines is somewhat sketchy. However, a number of rules
requires specific frequencies which havenotyet
been have been established theoretically and/or experimentally
produced coherently, and it also includes a table listing whichallow a qualitative prediction of the four most
all materials which have exhibited SRS to date, together
with the observed frequency shifts. A considerable num1 I n all cases where a comparison has been made between experiber of references are given which may serve as a guide for mental and theoretical values of the exciting light power density
necessary to reach threshold for SRS, the theoretical values were
deeper penetration into general Raman spectroscopy, as found to be considerably higher [20], [21]. (Thyy were computed
from
the measured scattering cross sections for mcoherent Raman
well as for anorientationamongthecurrentproblems
effect.) The discrepancies cannot be expressed by a constant factor,
in thefield of SRS.
but are different for different lines. Several mechanisms have been
suggested as causes [Zl],[22], but while one of them [22] was subseThespectralrange
which hasalready beencovered
quently shown t o be irrelevant [20], the others have not been tested
conclusively as yet. In spite of thjs situation, the peak intensity of
a n incoherent Raman line can still be consldered as the essential
guide for its expected SRS threshold, and in each case where, data
are available the rule has been followed that for any particular
substance the) incoherent line with thehighest peak intensity should
exhibit the lowest SRS threshold.
Manuscript received October 4, 1965. This work was supported
in part by the ElectronicTechnology Div., Avionics Lab., Research
and Technology UIV., Alr Force Systems Command, Wnght-Patterson Air Force Base, Ohio, under Contract AF 33(657)-11650.
The author is with Hughes
Research Labs., Malibu, Callf.
1
2
IEEE JOURiX.4L O F QUAXTUM ELECTKONICS
TABLE I
CHARACTERISTIC
FREQUENCIES
OF STRETCHING
important properties of Itaman lirles-frequencies, intensities, line widths, and depolarization- o n t,hc basis
of I) the influence of the chemicalbond and 2 ) t,he influence of t,hc symrnetzy of thc molecule and of the vibrat.iona1mode [%-[%]. h comparison of all coherent
Raman spectra obtained t o date with the normal Raman
spectra of the same substances clearly shows t,hc general
validity of these rules and establishes their applicability
as a guide in selecting Raman laser materials for a specific
purpose.
11.
PRINCIPLES FOR
[30], [31].
VIBRATIOKS [27],
Frequencg
(cm-1)
445-550
490-5’22
510-594
570-650
600-700
630-705
700-1100
750-850
SELECTING
RAMAN
LASER&h’l’ERIL4LS
A. InJluence of llze Chemical Bond on the Pmperties of
Raman IJznes
It has been determined by lZaman and infrared speetroscopy that certain chemical bonds or groups of atoms
exhibit characteristic vibrational frequencies which vary
within a r~lat~ively
small range when t,he group belongs
to various molecules. Table I shows a number of groups
of at,oms and the characteristic frequency ranges of
their stretching vibrations. The t,otal range est,ends from
445 to 4160 c m - l . h more detailed account of the characteristic frequencies is found in Brandlnuller and Nloser
[27], Jones and Sandorfy [30],and Mixushima [81].Table
I1 shows more specifically the changes in frequency one
can expect t o obtain if, for example, a NO, group is
at,tac:hed t o various st,ructures[32].In many cases solvents
affect t,hc vibratiord frequencies of dissolved molecules,
thus providing a convenient means for tuning a Raman
laser frequency. These frequency shifts, rangingup to
some tens of cm-’, are caused by complex formations in
caused by hydrogen bonding.
the liquid; the large shifts are
When a certain group is part of similarmolecules,
and when it is sufficiently isolated from anothergroup
of the same kind, its Raman lines possess not only charact.eristic frequencies, but characteristic intensit,ies as
well [ B ] , [as],[33]. The intensit,ies depend strongly on
t,he type of bonds between the vibrating aDoms. They are
usually muchhigherforcovalent,
than for ionic bonds.
This is caused by the fact that the Raman line intensity
resulting from a vibration of a bond is dependent 011 the
rate of variation of the polarizability during the vibrat,ion. In the case of a covalent bond, valence electrons
are shared by the atoms, and a change of the distance
between the nuclei will strongly affect t,he polarizability.
In the case of ionic binding, where each electron is essentially under the influence of only one nucleus; the
change of polarizability during a vibration will be small,
and accordingly, t,heintensity of theRaman linewill
also be small.
Investigat,ions of nunlerous organic compounds have
shomm that a great enhancementof the intensity of certain
Raman lines takes place(for excitationwith the commonlyusedvisible
or near ult;l.aviolet mercury lines)
884-899
939-1005
990-1050
1020-1075
1085-1125
1120-1130
1188-1207
= 1190
1216-1230
‘
= 1340
= 1380
= 1380
1590-1610
1610-1640
1620-1680
= 1630
1654-1670
1650-1820
1695-1715
1974-2260
2150-2245
~2570
2800-3000
3000-3200
3150-3650
3300-3400
4160
\%rating
Group
Type of
Compound
s-s
c-I
C-Br
c-c1
aliphatic disulfides
aliphatic comp.
aliphatic comp.
aliphatic comp.
C-SH
mercaptans
c-S
aliphatic comp.
c-c
aliphatic comp.
para derivatives o f
benzene
cyclopent,ane and
mono derivatives
cyclobutane and
i-~:
L .:
derivatives
benzene and mono- to
tri-subst. benzenes
c-O-c
aliphatic comp.
C-OH
aliphatic comp.
c=c==o
aliphatic comp.
cyclopropane and
/
.-_
derivatives
so2
aliphatic comp.
-S=o
aliphatic comp.
0
iIi
iI>
’/
/O
N .-..
\O
A 0
”&
a
C)
N=o
c=c
C=N
C=N
C 4
C 4
c=c
C=N
S-H
C-H
C-H
O-H
N-H
H-H
aromatic comp.
aliphatic comp.
naphthalene and
derivatives
benzene derivatives
aliphatic comp.
aliphatic comp.
aromatic comp.
aliphatic comp.
aliphatic comp.
aromatic comp.
aliphatic comp.
nit,riles
aliphatic comp.
aliphatic comp.
aromatic comp.
aliphat,ic comp.
aliphatic comp.
Hz
when the molecule has conjugatcd bonds (i.e., alte~xating
single and double bonds)or, more generally, when t’he
molecule exhibits ‘‘resonance”’ [%I,
[29]. Examples of
the effect are presented in Table HI. This shows the
intensity of Raman lines from the KO? and the C = X
groups, respectively, as part of 1) nonconjuga,ted and 2)
conjugated systems [as].
The following reason underlies this phenomenon.
Conjugation causes a considerable change of the electronic energy states of a moleculecompared
with a
similar one which has no conjugated bonds. It particularly
affects the low-energy excited staOes which are frequently
most important as intermediatestates for the Raman
This term is used in chemistry to describe the delocalization of
electrons. The word “resonance” indicates that a molecule cannot
be represented by a single classical structure with localized bonds
or charges, but only by several such structures, and its true state is
essentially a hybrid of these representations [34].
T
4
IEEEELECTRONICS
JOURNAL O F QUANTUM
JANUARY
+
localized T electrons as compared to other organic mole* b,2,(3aA2
+y’2)Io
(5)
cules exclusively containing u electron^.^
The closer the frequency of the exciting light approaches for irradiation with a collimated beam,and the degree
that of the first electronic absorption band of any molecule, of depolarization for linearly polarized exciting light, obthe more emphasized, in general, is the role of the ap- served perpendicularly to the direction of E and the diproached states as intermediate states for Raman scat- rection of incidence, becomes
tering. I n the case of resonance, the corresponding frequency difference term in (4) will become very large4 and
only the approached electronic state needs to beconsidered.‘ ResonanceRaman
effect [27], [29] has been Here, QICn is the total scattering cross section, I , is the
studied mainly on conjugated systems, and the intensity intensity of the exciting light,
of certain Raman lines was found to increase by several
orders of magnitude. The stimulated version of the resonance Raman effect has not been reported yet.
B. Injluence of the Symmetries of Molecules and Molecular
Vibrations on the PYopeTties of R a m a n L i n e s
I n general, the scat,tering tensor iscomplex andunsymmetric, and the sum which extends over all the excited electronic statesand
represents itscomponents
cannot be evaluated. An important simplification of this
problem was found by Placzek [24]. He showed that for a
nondegenerate electronic ground state and for a vo, which
is essentially different from any electronic or vibrational
eigenfrequency of the molecule, t,he scattering tensor is
real andsymmetricand is related t o the polarizability
tensor. Its componentscan then beexpressed as sole
functions of the vibrational coordinates yi. By a Taylor
expansion in these coordinates, Placzek derived expressions for the intensity and the degree of depolarization
of Raman lines, which contain only two invariants of the
polarizability derivative tensor a’:its “average” (or isotropicpart) ah andits“anisotropy”
y”. Under these
conditions, the Raman intensity due t,o scattering by a
single molecule becomes
For moleculespossessing the symmetry of one of the
common point groups, certain components of the polarizability tensor vanish, as was determinedwith the aid
of group theory. This result,s in some general rules concerning intensity and depolarixaOion for lines of certain
symmetries. It was found that for the t-otally symmetric
vibrations ah does not vanish, but that it does vanish for
all others (the rule is only approximately valid for degenerate vibrations). In general, therefore, thetotally
symmetric vibrations produce the most intense Raman
lines.
Fortotallysymmetric
vibrations of all molecules of
cubic symmetry (point groups Oh, T,, 0, T,, T ) y r 2 = 0,
and therefore, ps = 0 according t o (6). For all other
symmetry groups p s is not determined by symmetry and
varies between 0 and 2. For all nontotallysymmetric
vibrations
p = 2,since a; vanishes. The values for p which
3 Since some of the terms in
(4)may benegative, an “interference
of probability amplitudes” can occur. For many of the molecules in are quoted in the 1iOerature are in general too high for
question and the excitation frequencies as stated, the situation is strongly polarizedlines [41]. This results from the fact
much simplified, however, by the following findings: Experimental
studies on polyenes and on nitro-compounds [29], as well as theo- that they represent mean values because the depolarizaretical intensity calculations on polyenes and. some aromatics [37], tion of a Raman line varies as a function of frequency
have shown that for the strongest Raman
lines from these compounds only the lowest exclted states are essential as intermediate due t o its rotational wings: The center part (Q branch),
states, thus eliminating the sum overall the other states in
(4). which contains all the light scattered on account of the
For additional detailed theoretical treat’ment8 on this problem of
theinterference
of probability amplitudes, see Behringer and isotropic part of the polarizability derivat,ive tensor, is
Brandmuller.[29], Behringer [38],and Albrecht [3!].
more strongly polarized.
4 A dampmg correctlon must be added to (4) If resonance occurs
In the case of crystals, it is customary to distinguish
[381.
6 For some simple molecules or ions (e.g., CCL, SO2; CO,z;),
between internaland external (lattice) vibrations. The
all Raman lines were found to be enhanced in case of resonance with
the first absorption bands [40]. However, this is not the case for all former are vibrations which take place m-ithin groups of
molecules. For conjugated molecules, mainly Raman lines due to
atoms which are more tightly bound t,o each other than
totally symmetric vibrations have been enhanced in intensity when
to otheratoms or goups in the crystal, e.g., COi- in
the first absorption bands were approached [27].
Experimental evidence was presented by Shen and Shaham and calcit,e or any molecule ina
molecular crystal. The
by Lallemand and Bloembergen that the anoma1:es inthe SRS
thresholds result forma self-focusing effect of the exciting laserbeam totally symmetric ones among these vibrations generally
in the Raman activeliquid. (Y. R. Shen and Y. J. Shaham, “Beam show little dependence of their properties on the phase
deteriorationandstimulatedRaman
effect,” Phys. Rev.Letters,
vol. 15, pp. 1008-1010, December 27, 1965; P. Lallemand and N. of the material, provided that the compound exists in a
Bloembergen, “Self-focusing of laser beams and simulated Raman free state. Very detailed calculations have been made on
gain
in
liquids,” Phys. Rev.
Letters,
vol. 15, pp. 1010-1012,
absolute intensities and depolarizations by Saksena [42]
December 27, 1965.)
1966
5
ECKHARDT : RAMAN LASER MATERIALS
for all modes of lattice vibrations in the 32 crystal classes
for specified directions of irradiation.
Thewidth of vibrationalRaman linesis essentially
determined by rotational motions of the molecule as a
whole or some of its parts, by intermolecular interactions
(in liquids and solids), by the occurrence of structurally
similar, weakly coupled groups which are not located in
equivalent positions within the molecule, and by isotope
effects. The complexity of these effects explains the lack
of a general theory for the widthof Raman lines in liquids
and solids.
It was found experimentally thattotally symmetric
vibrations usually produce the sharpest lines in any one
spectrum. This can be understood by means of a detailed
theoretical discussion of the rotational Raman effect [43].
According tothistheory,
only the anisotropy of the
polarizability derivative tensor is responsible for the
rotationalstructure (or therotational wings) of vibrational lines. Consequently, lines with vanishing or small
7‘’(totally symmetricvibrations) are usually very narrow.
Thetheory of therotationalRaman
effect applies
strictly only to gases, but it is &illpossible to make
some qualitative statements for the case of a liquid or a
solution. The following two effects have the most profound influence on the widths of Raman lines from liquids:
the random fluctuations of the intermolecular forces and
the formation or dissociation of complexes[44]. Both
effects will change the rotationalmobility of the molecules
and can, therefore, increase as well as decrease the line
width. Again, the influencewill
bemostpronounced
for nontotally symmetric vibrations and will hardly, or
not a t all, affect thetotally symmetricvibrations for
which y” is small or vanishes [45], [46]. However, if a
group participates in a hydrogen bond, even its totally
symmetricvibrational line maymarkedlychange
its
width [45].
siderable number of cases, CH, (n = 1, 2, 3, 4) vibrations
could be stimulated. These vibrations are known to produce intense lines because of their large zero-point amplitude which is due to the small mass of the H atom (this
applies to HZ as well) and also because of someproportionality existing between the intensity and number
of participating bonds (for n > 1) [50]. I n most of the
listed organic compounds, several CH, groups exist per
molecule; however, since the groups are weakly coupled,
they do not add to the peak as much as to the width of
the line. The compounds CCI, and CH C1, are weaker
scatterers than those with a conjugated system, as can
be seen from Table V. This table lists some of the SRX
materials, together with relative intensity values of the
strongest Raman line of each. The intensity is expressed
by a scattering coefficient S (also calledstandard intensity)
relative t o the 459 cm-’ line of liquid CCl,, as defined by
Bernstein and Allen [51]:
+
77’7
(45ff’2 77’2)459*
X = (45&
+
The absolute scattering cross section for a Raman line
can be calculated from the scattering coefficient X, since
it has been determined that ( 4 5 ~ 1 ’ ~=) ~13
~ ~X lo-’
cm4g-l for CCl, vapor [60]. Using expressions from [24]
and [23], together with (5) and (7), the following approximate relation between the scattering coefficient S
andthe tot,alscattering cross section Q for forward
scattering per molecule and per unit solid angle is obtained for irradiation with a linearly polarized collimated
beam?
111. DISCUSSION
OF MATERIALS
AND
Here, %, and i; are the respective wavenumbers of the
VIBRATIONS
EXHIBITING
SRS
exciting light and molecular vibration, g is the degree of
If oneconsiders the substances and the participating
degeneracy of the Ramanline; and no,the refractive index
vibrations which have been found t o exhibit SRS to date of the particularsubstance at thefrequency of the exciting
(Table IV), one finds that there are representatives of a light. The factor ((ni 2)/3)4 is a local field correction,
large variety of classes of materials among those listed:
different kinds of aromaticcompounds
(e.g., benzene
The absolute value of cy’z for the 459 cm-1 Raman line of CCla
derivatives and naphthalene derivatives), saturated
six- has6 been
measured from the liquid [61] as well as from the vapor.
by measuring relativetothe
membered rings, different kinds of aliphatic compounds The liquidvaluewasdetermined
Rayleigh line and the vapor value by
measuring relative to the pure
(e.g., ketones and dienes), inorganic compounds, and rotational line of hydrogen (J = 1 + 3) [60]. The two values for O L ’ ~
obtained in this manner differ by a factor 2.6. Since the theoretical
elements,
for Raman and Rayleigh scattering apply to a gas only,
The organic compounds wereused either as liquids, formulas
a number of corrections must be made when they are applied to
scattering
in a liquid. All corrections for the Raman line apply for
solutes, or gases, the inorganic compounds ascrystals
the Rayleigh line as well and, therefore, are cancelled out in a comor liquids, and the elements as crystals, liquids, or gases. parative measurement. However, Rayleigh scattering intensities are
All vibrations which werefound active in SRS are stretch- affected in addition by interference effects and Tyndall scattering,
and the corrections for these effects contain a large uncertainty [60].
ing vibrations, withthe exception of one S8 vibration Thus,
the value for a’2obtained from the vapor can be considered
(216 cm-’)[32]. Theyare all of the totally symmetric as more accurate, and should, therefore, be used in (8). The use of
a n a’s value determined from vapor measurements ignores the in-kind. Many of the groups which participateinthese
fluence of intermolecular interactionsinthe liquid. However, in
vibrations are either completeconjugatedsystems (one most cases, this influence can be expected t o befairly small for
totallysymmetric vibrations, and scattering cross sections calcuaromatic ring or two condensed aromatic rings) or parts lated
in this way from scattering coefficients agree quite well with
of conjugated systems C = C, C=N, and NO,. I n a con- those quoted in Bret and Mayer[21] and McClung andWeiner [56].
+
6
JANUARY
IEEE JOURNAL O F QUANTUM ELECTRONICS
TBBLE I v
S u ~ s r a s c s FOUND
s
TO EXHIBIT
SRS
Substance
a-sulfur
carbon tetrachloride
a-sulfur
carbondisulfide
chloroform
orthoxylene
calcium tungstate
deuterated benzene
benzene
pyridine
aniline
styrene
bromobenzene
benzonitrile
ethyl benzene
toluene
fluorobenzene
calcite
styrene
diamond
nitrobenzene
1-bromo-napthalene
Vibrating
Group*
Observed
Flmdamental
Frequency
Shifts,** (cm-l)
Reference
216
460
470
655
663
730
911
944
990
992
997
999
1000
1002
1002
1004
1009
1086
1315
1332
1344
1363
SS
c-Cl
SS
css
c-CI
1-chloro-naphthalene
1368
naphthalene
1380
Substance
oxygen
cinnamaldehyde
styrene
3-methylbutadiene-1, 3
pentadiene-1, 3
benzonitrile
nitrogen
tetrahydrofuran
cyclohexane
cyclohexanone
orthoxylene
methane
acetone
metaxylene
piperidine
tetrahydrofuran
cyclohexanone
I, 1, 2, 2 t e t m
chloroethane
deuterium
paraxylene
styrene
benzene
hydrogen
mesitylene
Observed
Fundamental
Frequency
Shifts,** (cm-l)
Vibrating
Group*
0-0
1552
1624
1631
c=c
c=c
c=c
C=C
C-N
N-N
CHs
CHs
CHz
CH,
CH,
CH3
CH3
CHs
CHz
CHs
C-H
D-ll
C-H
C-H
C-H
H-H
Reference
1
I
I
1
1638
1655
2229
2327
2849
2852
2863
2913
2916
2921
2933
2933
2939
2945
2984
2991
2998
3056
3064
4155
* The assignments are according to Landolt and Bornstein 1321, except for pyridine [47], styrene [48], and all monoderivatives of benzene [49].
** The uncertainty of the frequency shifts can be as large as f 1 0 cm-1. I n some cases, the shifts quoted in the references were replaced
by more accurate results obtained by the present author.
t These laser lines appeared as multiplet. Only the smallest shiftis given [3].
TABLE V
S, DECREEOF DEPOLARIZATION
pa, AND LINEWIDTH
6 FOR SOME
SCATTERING COEFFICIENT
INCOHERENT
RAMAN LINES, WHICH HAVEPRODUCED
BEEN COHERENTLY
Substance
Vibrational
Frequency
(cm-l)
s
carbon tetra-chloride
chloroform
459
668
1.00
0.669
benzene
992
&benzene
945
4.04
1004
toluene
6
Ref.*
Pa
Ref.
(cm-1)
0.0065
0.092
determined
from [57]
0.028
1.8
2.816
0.01
1.5
6.9
nitrobenzene
1345
5.27
0.22
methane
hydrogen
deuterium
2914
4160
2993
3
1.38}
.0
0.00
Ref.**
1541
1.16
* Many scattering coefficients have been measured repeatedly. See also Brandmuller and Moser [27], Jesson and Thompson [58], and
Marckmann et al. [59].
** The linewidths according t o Brandmuller and Schrotter [54] are directly measured.
1966
LASER
ECKHARDT RAMAN
which accounts for the change of the applied radiation field
when acting upon a molecule in a substance of refractive
index no[67].
Most substances which have shown SRShave been
in the liquid phase, and only four crystals are among the
listed SRS materials. The following reasonsmay be pointed
out t o explain the fact: There are very few crystal structures which are compatible with the demands of covalent
compounds regarding valency and spatial configuration.
The most important ones among these are the diamond,
zincblende, andwurtzitestructures,but
only diamondtype crystals have purely covalent bonds. (The 111-V
compound semiconductors, for example, ’ which crystallize in the zincblende lattice, and the 11-VI compounds,
which crystallize either inthe zincblende or wurtzite
lattice,havepartially ionicbonds. Therefore, they can
be expected to beweaker scatterersthan diamond or
silicon.) It follows from the above argument that,organic
compounds form, in general, molecular crystals whose
external (lattice) vibrations are not very strongbecause of
the weakness of their intermolecular bonds. The internal
vibrations of these crystals,. on the other hand, are only
slightly shifted in frequency and little altered in intensity
by the transitionfrom the liquid to thesolid phase. Therefore, as long as one does not go to very low temperatures
where the linewidths can be expected to decrease, little
incentive exists to use molecular crystals for SRS, rather
than the corresponding liquids. At power levels similar to
those required t o achieve SRS from internal vibrations of
covalent complexes in’calcite (COi-) and calcium tungstate (WOi-), some other crystals containing covalent
complexes should also exhibit SRS.
It has been observed that crystals show surface damage,
damage tracks,or cleavage whensubjected to alaser beam
of a power density in the multi-MW/cm2 range.’ The
damage has beenascribed to acoustical phonons generated
by the laser beam [62],[63], but is not yet fully understood. (See also Hercher [64] and Chiao et al. [65].)
Under the same circumstances, several organic liquids
have shown evidence of some photochemical reactions,
such as a change of color or the appearance of numerous
small suspended particles. No investigations of these processes occurring in liquids have been reported as yet. The
effects mentioned here mayrestrict
considerably the
number of suitable materials for any application requiring their repetitive use, and they also set an upper
limit for the power densitywith which aparticular
liquid or solid can be irradiated repeatedly.
I n conclusion, it can be stated that it is only a question
of the achievable power level of the exciting laser and of
the chemical stability and/or mechanical strength of the
compound whether a Ramanactivesubstancecanbe
stimulated to exhibit Raman laser action. However, for a
highconversionefficiency
a t typical excitation power
7 Tracks and surface pitting have been produced in calcite and
a-sulfur while at the same power level, diamond showed a phase
transistion t o graphite on the surface, but no tracks; cleavage was
observed in naphthalene [6].
7
hfATERIALS
levels (0.5-50 MW), one should choose compounds with
strong absorption bands close to the exciting frequency,
inparticular conjugatedcompounds for excitation in
the visible or near ultraviolet; also, for a restricted frequency range, compounds containing CH groups, and
more generally, groupswith strongly covalent bonds.
One should anticipate the most success with the stimulation of totally symmetricstretching
vibrations for
any given compound.
ACKNOWLEDGMENT
It is a pleasure to acknowledge stimulating and helpful
discussions with S. Yatsiv, J. D. Margerum, W. G.
Wagner, and H. V. Winston.
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