Volume
183,number 5
CHEMICAL PHYSICS LETTERS
A molecular beam millimeter-wave optical pump/probe
of the X ‘E+ state of yttrium monofluoride
6 September 199 I
study
J.E. Shirley, W.L. Barclay Jr., L.M. Ziurys ’ and T.C. Steimle
Departmenl of Chemistry, Arizona State University,Tempe, AZ 85287-1604, USA
Received 6 May 1991; in final form 3 June 1991
The J= 4-3, J= 5-4, and J= 6-5 pure rotational transitions in the ground X ‘Z+ state of yttrium monofluoride, YF, have been
recorded by molecular beam millimeter-wave optical pump/probe spectroscopy. This study demonstrates that high resolution
( < 100 kHz fwhm) millimeter-wave measurements of high temperature refractory compounds can readily be obtained via the
combination of optical detection and high temperature effusive oven molecular beam sample preparation. The measurements
performed here provide an improved determination of the ground state rotational constants 8=8683.65 (1) MHz, and D=O.O079
(2) MHz.
1. Introduction
In the continuing study of transition metal containing compounds it has been shown that there is
need for increasingly higher resolution in order to
reveal small splittings (e.g., n-doubling, magnetic
hyperline, electric quadrupole, Stark shifts, etc.) that
provide the valuable information. Thus, in our laboratory a molecular beam millimeter-wave optical
pump/probe (MODR-P/P) spectrometer similar to
those of Rosner et al. [ 11, Childs and Goodman [ 21,
Ernst and Kindt [ 31, and Kniickel et al. [4] has been
constructed. In this technique the molecular beam
sample is sequentially exposed to laser pump beam,
millimeter wave and a laser probe beam radiation.
In the experiments described here the laser pump and
probe beam wavelengths were identical. A millimeter-wave transition is detected by monitoring the
probe beam laser-induced fluorescence (LIF) as a
function of swept millimeter-wave frequency. The
technique is the optical analog of the conventional
molecular beam electric resonance experiment [ 5,6].
The X ‘E+ of YF is a convenient choice for an initial application of the MODR-P/P technique to high
temperature refractory compounds because of the
absence of Zeeman, magnetic hypertine and electric
’ Presidential Young Investigator.
0009-2614/91/$
quadrupole effects. Thus, it is expected that each pure
rotational transition will consist of a single feature.
Furthermore, the previous molecular beam measurements [ 71 demonstrated that strong LIF signals
could easily be achieved from the visible B ‘II-X ‘Z+
band system of this molecule.
2. Experimental
A block diagram of the experimental arrangement
is given in fig. 1. As in the previous study [ 71, YF
was generated by heating a tantalum crucible containing YFJ and aluminum chips to z 1600 K by
electron bombardment. The effusive gas was then
collimated by two 4 mm apertures placed 5 cm and
20 cm from the crucible to produce LIF spectral features with a residual Doppler linewidth of z 35 MHz
(fwhm). The LIF was detected, via photon counting, through a lens assembly and interference bandpass filter (fwhm = 10 nm) centered at 630 nm. The
laser raditition was obtained from a commercial cwring dye laser system operating in the DCM dye
region.
The millimeter-wave source consisted of two tunable Gunn oscillators with a combined frequency
range of 84-l 15 GHz and maximum output power
of 30-80 mW. The output frequency of the Gunn os-
03.50 0 1991 Elsevier Science Publishers B.V. All rights reserved.
363
Volume 183, number 5
CHEMICAL PHYSICS LETTERS
6 September 199 1
I
C
TERtll
COMPUTER
NRL
I
-@
I
, PROBE
LASER
PLOTTER
I
ISOLATOR
I
CUNN
OSC.
IE
DIRECTIONALCOUPLER
’
I
-
ATTENUATOR
I-
HRRnoN’C
IIIXER
-
-
; HORN
CORRECT
1 ON
VOLTAGE
i
FREQUENCY
COUNTER
n
,
c
I
1 PUtlP
FREQUENCY
SYNTHESIZER
<@-2CHx
-
TRIPLEXER
)
I00
flHz
IF
1
LRSER
REFERENCE
PHASE
OSCILLRTOR
LOCK
BOX
II0
ITOR
IIOLECULRR
IF
BEAtl
Fig.
I. A block diagram of the MODR-P/P spectrometer.
cillator was stabilized to x 1 kHz by mixing it with
a harmonic of a synthesizer operating near 2 GHz.
A resulting 100 MHz-IF signal was phase-locked to
a 100 MHz reference oscillator by adjusting the Gunn
oscillator bias voltage. Frequency scans up to 10 MHz
in range were accomplished by sweeping the synthesizer frequency and allowing the phase-lock circuit to readjust the Gunn oscillator bias voltage.
As shown in fig. 1 the molecular beam is optically
pumped (Z 150 mW laser power) in region A and
then probed ( z 1 mW laser power) with radiation
of the same frequency in region C. The Q( 4)
(u=15885.284cm-‘),
R(5) (v=15888.245cm-I),
and R( 6) (v= 15888.495 cm-‘) optical transitions
of the B ‘II-X ‘Z+ (0,O) band system were used in
the optical pump/probe detection scheme for the
J” ~4-3, J” = 5-4 and J” = 6-5 pure rotational transitions of the ‘Z+ (v=O). The optical features were
readily identified with aid of the previous Doppler364
limited analysis of Barrow et al. [ 81 and Kaledin and
Shenyavdkaya [ 9 ] . Under typical operating conditions the photon signal (counts/s) for the R( 6) feature, for example, are: oven background=2000;
pump laser scatter = 600; probe laser scatter= 1600;
LIF without pump laser = 6 100; LIF with pump laser
= 2050. This corresponds to x 70°h optical pumping
efficiency. The millimeter-wave radiation was introduced into region B through an E field 25 mmx 5
mm horn antenna.
When tuned to the resonant millimeter-wave frequency an increase in probe beam LIF is detected.
Fig. 2 shows a representative spectrum of the J” = 65 pure rotational transition corresponding to ~25
FW of millimeter-wave power and 15 min of signal
averaging. The transit time broadening was calculated to be z 1 kHz for the molecular beam travelling at 560 m/s (corresponding to (velocity) of YF
at 1600 K) over a microwave radiation field esti-
Volume 183, number 5
CHEMICAL PHYSICS LETTERS
6 September 1991
Table 1
Millimeter-waveobservationsand spectroscopicconstantsfor the
X ‘Z+ (~0) state of YF
Rotational
transition
Obs. freq.
(MHz)
Calc. freq.
(MHz)
Diff
(kHx)
4t3
5+4
6t5
69467.2095
86832.5969
104197.0200
69461.2113
86832.912
104197.0228
- 1.8
5.7
-2.8
Spectroscopicparametersp)
104196.8ao
104i97.014
Frequency (MHz)
Fig. 2. The J” -6-5 pure rotational transition of the X ‘Z+
(u=O) state of YF. The R(6) (0,O) B ‘D-X ‘Z+ branch feature
was used in the opticaCpumping/LIFdetection scheme. The signal represents approximately I5 min of averaging.
mated to be approximately 6 cm long. The power
( x 25 uW) broadened linewidth was calculated from
the Rabi flopping frequency expression [ 10 ] :
Av=~x(,u*E*~-~)
I’*=99 kHz,
(1)
where p is the permanent electric dipole moment and
E is the millimeter-wave electric field strength. The
value of p= 1.82 D was taken from the results of the
previous molecular beam study [ 71. The experimental linewidth of 96 kHz (fwhm) is consistent
with the estimated power broadening and also the
observation that the linewidth was reduced to z 20
kHz, at the considerable reduction of signal-to-noise,
by reducing the millimeter-wave power.
3. Results
The results of a weighted linear least squares fit of
the molecular constants Band D to the experimental
data are given in table I. The simple expression, appropriate for an unperturbed 5’ state,
E=B”J(J+l)-D”J2(/+1)Z
B (MHz)
D (MHz)
8683.65(I )
8682.0
8684.9(3)
0.0079(2)
104197.162
(2)
was used to calculate the transition frequencies. The
measured frequencies were equally weighted. A comparison of the determined molecular constants to
previous values is also given in table 1. The B constant determined by the present analysis of the pure
rotational spectra is different from that extracted
this work
ref. [8]
ref. [9]
0.0074(2)
‘) The numbers in parentheses represent a 2a error estimate.
from the optical analysis [ 91 by more than 10 standard deviations given for that determination. This is
probable due to the correlation of the parameters derived from the optical analysis.
4. Conclusion
A millimeter-wave optical pump/probe spectrometer has been constructed that can be used in conjunction with high temperature (T> 1600 K) molecular beams to produce spectral features with
linewidths of z 20 kHz. This technique will allow for
ultra-high resolution studies of other refractory systerns. The necessity of only minute millimeter-wave
power makes this approach well suited for the study
of high frequency transitions where harmonic generation of the radiation is required. It is also ideal for
the study of weak millimeter-wave transitions such
as magnetic dipole allowed transitions.
Acknowledgement
This work was supported by grants from the National Science Foundation (TCS-CHE-9022073 and
LMZ-AST 9058467 ).
References
[ I] S.D. Rosner, R.A. Holt and T.D. Gaily, Phys. Rev. Letters
35 (1975) 785.
[2] W.J. Childs and
1214.
L.S.
Goodman. Phys. Rev. A 21 (1980)
365
Volume 183,number 5
CHEMICALPHYSICSLETTERS
[3 ] W.E. Ernst and S. Kindt, Appl. Phys. B 31 ( 1983) 79.
[4] H. Kniickel, T. Krockerskothem and E. Tiemann, Chem.
Phys. 93 (1985) 349.
[5] N.F. Ramsey, Molecular beams (Oxford Univ. Press,
Oxford, 1963) .
[6] W.J. Childs, Case Stud. At. Phys. 3 ( 1973) 2 17.
366
6 September I99 1
[7] J.E. Shirley, CT. Scurlock, T.C. Steimle, B. Simard, M.
Vasseurand P.A. Hackett, J. Chem. Phys., in press,
[8]R.F. Barrow, M.W. Bastin, D.L.G. Moore and C.J. Pott,
Nature215 (1967) 1072.
[ 91 L.A. Kaledin and E.A. Shenyavdkaya,Mol. Phys. 70 (1990)
107.
[ IO] W. Demtriider, Springer series in chemical physics, Vol. 5.
Laser spectroscopy (Springer, Berlin, 1982).