Laser-Stark spectroscopy of fluoromethane in a molecular beam

JOURNAL OF MOLECULAR SPECTROSCOPY 101,

325-33 1 (1983)

Laser-Stark Spectroscopy of Fluoromethane in a Molecular Beam C. DOUKETIS

AND

T. E. GOUGH

The Centrefor Molecular Beams and Laser Chemistry,’ Department of Chemistry, Universityof Waterloo, Waterloo,OntarioN2L 3G1, Canada The laser-Stark spectrum of the Y)vibration of fluoromethane in a molecular beam is recorded using bolometric detection methods. The resolution of the spectra is limited by the homogeneity of the applied electric field to one part in 3000 of the Stark shift required to tune a transition into resonance. The observation of several field induced transitions of fluoromethane is reported. INTRODUCTION

The molecular beam-bolometric method of recording sub-Doppler resolution infrared spectra is now a recognized alternative to saturation spectroscopy (I, 2). To date, the molecular beam-bolometric method has used tunable diode and color-center lasers to excite vibrational transitions in the molecules of the beam: these lasers make the method highly flexible, avoiding the necessity for coincidences between molecular absorptions and laser emissions. Nevertheless, the higher powers available from fixed frequency lasers, sufficient to saturate most vibrational transitions, can improve the sensitivity of the molecular beam-bolometric method. Furthermore, because the amplitude of the saturated signal is independent of laser power, and of transition probability, quantitative measurements of the populations of particular, rovibrational quantum states of the molecular beam are greatly simplified. In this paper we wish to describe an extension of our original molecular beambolometric method: a carbon dioxide laser is used as excitation source and an electric field is used to tune molecular rovibrational transitions into resonance with this laser. The molecule chosen to evaluate the technique was fluoromethane, which has been the object of an exhaustive laser-Stark investigation by Oka and co-workers (3). This investigation showed that v3transitions of fluoromethane may be observed originating from every rotational level which is expected to be populated in a supersonic molecular beam. The experimental apparatus is very similar to that used to measure the vibrational dependence of the dipole moment of hydrogen fluoride (4), and the vibrational dependence of the polarizability tensor of carbon dioxide (5). Such measurements were made by observing second-order Stark splitting using a static electric field and a tunable color-center laser. For the experiments reported here, much larger Stark shifts were used and, therefore, more homogeneous and stable electric fields were ’ An interdisciplinary Center whose members participate in the Guelph-Waterloo Program for Graduate Work in Physics, the Guelph-Waterloo Center for Graduate Work in Chemistry and the Guelph-Waterloo Surface Science and Technology Group. 325

0022-2852/83 $3.00 CopyrIght All

nghts

8

1983 by Academic

of reproduction

Press.

in any form

Inc resenrd

326

DOUKETIS AND COUGH

required in order not to degrade the resolution of the experiment. Laser-Stark spectroscopy of molecular beams has been performed by Shimizu et al. on ammonia (68) and fluoromethane (8). In these investigations the spectra were obtained by observing the attenuation of the laser radiation by the molecular beam. This technique, which is far less sensitive than bolometric detection, necessitated the use of an effusive multichannel beam source. The resolution attainable with this approach is inferior to that reported here. EXPERIMENTAL

DETAILS

The molecular beam spectrometer used has been previously described (4). It was modified by the inclusion of a set of aluminum Stark electrodes surrounding the region of intersection between laser and molecular beams. The electrodes were circular and, therefore, could be machined on a lathe. No polishing of the electrode surfaces was performed, the only critical criterion being the parallelism of the electrode surfaces and the surfaces in contact with the supporting annular spacer. This spacer was made on a lathe from machinable ceramic. Slots were machined into the ceramic support to allow entrance and exit of the laser and molecular beams. The electrode spacing was varied by machining electrodes with different values of A (Fig. 1): electrode spacings between 0.07 and 0.5 cm have been successfully used. The maximum electrode diameter is approximately 10 cm. The Stark field was developed by applying the output of a Venus H-30 0- to 30-kV power supply across the electrode assembly. The output of this supply is controllable by a 0- to 13-V trim voltage, and it was this voltage that was biased and ramped in order to display a laser-Stark molecular beam spectrum. A variable potential divider was connected across the plates and adjusted until its output voltage for any chosen resonance of fluoromethane corresponded to the appropriate resonant electric field taken from the literature (3). Stark-tuned rovibrational transitions were excited by the output of a single mode line-tunable carbon dioxide laser. The output coupler of the laser was given a curved final surface so that the emitted radiation was focused to a beam waist (w,-,= 3.4 mm) at the point of beam intersection. The cavity support of the laser was constructed from stainless steel tubing through which water thermostatted to +0.02” was circulated. No active frequency stabilization of the laser was employed. For most of the transitions

.

BEAM

L___.i

t

-

B

FIG. 1. Cross section of the Stark cell for molecular beam laser-Stark spectroscopy. The shaded regions represent the annular ceramic spacer, and the laser radiation enters the cell through the dotted slit. The electrode spacing is varied by using different values of A.

LASER-STARK SPECTROSCOPY OF MOLECULAR

BEAMS

321

observed, the resolution of the apparatus is limited by the homogeneity and stability of the Stark field (1 in 3000). The plane of polarization of the laser was perpendicular to the Stark field so that only AM = +l transitions are observed in this work. RESULTS AND DISCUSSION

We present here results on the laser-Stark spectroscopy of molecular beams of fluoromethane seeded in helium. A separate paper (9) will describe the application of the technique to the characterization of rotational temperatures for such beams. In this paper we concentrate on the more spectroscopically interesting aspects of the work. Figure 2 shows the laser-Stark spectrum of fluoromethane recorded using the 9.4pm P( 18) carbon dioxide lasing transition and varying the electric field from 20 to 25 kV cm-‘. The electrode diameter (B in Fig. 1) was 5 cm. The fluoromethane was expanded as a 1% seed in helium through a 35-pm nozzle with POD, = 3.2Torr cm.

100 P!w

I

L

_/

AM

AM

AM

I

I

20

21 O---+-I I

I7207

I

I

22

23

I

24

I

25

KV-CM-’

-27

-IjO

_3T::_i-‘_,70 OT;

IT2

‘T’

Q(2,)

Q(33)

FIG. 2. A portion of the laser-Stark spectrum of fluoromethane in helium expanded through a 35-pm nozzle (POD, = 3.2 Torr cm.) The vertical scale has been converted to watts detected by the bolometer. The carbon dioxide laser was operating on 9 P( 18).

328

DOUKETIS AND COUGH TABLE I

Observed Resonance Conditions for Field-Induced Components of the yj Transition of Fluoromethane Transition

M Component

Laser Line

E kV cm-'

Q(l ,O)

O+tl

9 P(18)

20.60

S(O,O)

il+O

9 P(12)

38.02

O(i,O)

O+tl

9 P(24)

41.89

The laser power was sufficient to saturate and broaden somewhat the spectral transitions, with the exception of the largest peak at 20.6 kV cm-‘. Spectra recorded for gaseous samples of fluoromethane were dominated by a sextet, attributed to Q(3, 3), which masked the anticipated quartet of Q(2, 1); in addition an unassigned transition was reported to occur at 25.25 kV cm-’ (3). Examination of Fig. 2 clearly shows the Q(3, 3) sextet, now a minor feature because of the low rotational temperature of the beam, the Q(2, 1) quartet, to which the previously unassigned transition obviously belongs, and a previously unreported transition at 20.6 kV cm-‘. Schwendeman (10) has made a rather complete calculation of expected laser-Stark transitions of fluoromethane, using the molecular constants determined by Freund ef al. (3). These calculations confirm the above assignment of the Q(2, 1) quartet, and further indicate that the singlet at 20.6 kV cm-’ is to be assigned as the fieldinduced Q( 1, 0) A4 = 0 - + 1 transition. This transition, forbidden in zero electric

LASER

POWER

hlW

1

FIG. 3. Bolometer signal for several rotational components of the ~3transition of fluoromethane plotted against the square root of the laser power. The laser power may be approximately converted to W cme2 by multiplying by 10.

LASER-STARK

SPECTROSCOPY OF MOLECULAR

BEAMS

329

field, becomes allowed in finite electric field because of the mixing of adjacent J states. Such mixing should also allow the observation of AJ = rt2 transitions, given sufficiently close coincidence with carbon dioxide lasing transitions. @2,0) and S(0, 0) transitions are displaced minus and plus 6B from w. and therefore should roughly coincide with P(3, K) and R(2, K) transitions of fluoromethane, respectively. Table I also shows resonant voltages and carbon dioxide lasing transitions used to observe the o(2, 0) and S(0, 0) field induced transitions of fluoromethane. The O(2,O) transition appears as unassigned in the gas phase work of Freund et ui. (3), while the S(0, 0) transition was masked by a component of R(2, 1). In Fig. 3 we present data showing the power dependence of the bolometrically detected laser induced signals for selected transitions of the u3band of fluoromethane. For these studies the electrode diameter (B in Fig. 1) was 5 cm. Larger values of B improved the resolution of the spectra (see below) but under these conditions plots such as those in Fig. 3 were complicated by Rabi oscillations (II); as shown in Fig. 3 the decreased resolution causes the transitions to saturate smoothly. The curves for P(2, 1) and Q(2, 1) have identical high power asymptotes as is to be expected for transitions originating from a common lower level. The remaining curves in Fig. 3

AM= TEgE;;AL

2-3

(6)

l-2

(IO)

O-l

(12)

-1-o

(12)

FIG. 4. Laser-Stark spectrum of the Q(3, 2) component of the v1 transition of fluoromethane. Nozzle conditions as for Fig. 2, laser line 9 P( 18).

330

DOUKETIS AND GOUGH

may be used to confirm the assignment of the field-induced “forbidden” transitions. The common high power asymptotes of P( 1, 0) and Q(1, 0) and of R(0, 0) and S(0, 0) are clearly shown as is the much slower saturation of the field-induced transitions. Further confirmation of the assignments was obtained by measuring the variation in signal intensity while the nozzle pressure was varied. Transitions originating from common rovibrational levels have intensities which optimize at the same nozzle pressure. Figure 4 shows the molecular beam laser-Stark spectrum of the Q(3,2) component of the u3transition of methyl fluoride. This spectrum with its closely spaced transitions represents a useful check on the experimental resolution of the experiment. The spectrum in Fig. 4 was recorded with B (Fig. 1) = 10 cm, and with less laser power than that used for Fig. 2, so as to avoid broadening of the transitions. The relative intensities listed are calculated values (12) and it is clear from the experimental spectrum that partial saturation of the spectrum is present. The stick diagram below the spectrum shows the resonant fields predicted by the calculations of Schwendeman (10). The observed transitions have a full width half maximum of 14 V which corresponds to 2.1 MHz. The two closest transitions (M = -1 - 0 and -3 + -2) are calculated to be 11 V (1.6 MHz) apart. This spectrum shows that the electric field is homogeneous to 1 part in 3000 over the intersection volume of the molecular and laser beams. This homogeneity is attained in a readily demountable system without

TABLE II Electric Fields Necessary to Tune v3 Transitions of Fluoromethane into Resonance with the 9 P( 18) CO2 Lasing Transition E kV cm-' Transition

M Component

Q(l ,‘J)

O+il

20.571

20.60

Q(2,l)

l-2

20.755

20.79

O+l

21.279

21.32

-l+O

22.384

22.41

(al

(b)

25.21

-2+-l Q(3,3)

(c)

- 3+-2

20.320

20.314

20.30

-2+-l

20.906

20.919

20.91

21.543

21.544

21.54

O+l

22.235

22.192

22.22

1+2

22.937

22.925

22.93

2+3

23.684

23.670

23.68

-l+

(a)

Reference (3)

(b)

Reference (13)

(c)

present work.

0

LASER-STARK

SPECTROSCOPY OF MOLECULAR

BEAMS

331

polishing of electrode surfaces or adjustment of the parallelism of the electrodes. This simplicity of construction and operation is made possible by the relatively small intersection volume of the molecular and laser beams. APPENDIX

The referee of this paper has made available to us hitherto unpublished observations of the transitions shown in Fig. 2 recorded using Lamb dip saturation techniques on a bulk gas sample. In Table II we make comparison between the results of the original experiments of Freund et al. (3), the Lamp dip experiments (13) and the present molecular beam measurements. No attempt was made in the present work to lock the laser, either to a CO* fluorescence or to its gain curve; likewise no efforts were made to ensure orthogonal crossings of the molecular and laser beams. Hence the electric fields required for resonance could be in error by as much as 1 in 1000. ACKNOWLEDGMENTS This work was funded by an operating grant from the Natural Sciences and Engineering Research Council of Canada. R. H. Schwendeman provided us with invaluable assistance in the assignment of the fieldinduced transitions. RECEIVED:

April 7, 1983 REFERENCES

I. T. E. GOUGH, R. E. MILLER,AND G. SCOLES,Appl. Phys. Letr. 30, 338-340 (1977). 2. T. E. GOUGH AND G. SCOLES,“Laser Spectroscopy V” (A. R. W. McKellar, T. Oka, and B. P. StoichelT, Eds.), p. 337, Springer-Verlag, Berlin, 1981. 3. S. M. FREUND,G. DUXBURY,M. R~MHELD, J. T. TIEDJE, AND T. OKA, J. Mol. Spectrosc. 52, 3857 (1974). 4. T. E. GOUGH, R. E. MILLER,AND G. SCOLES,R. Sot. Chem. Farnday Discuss. No. 71, p. 77 (1981). 5. T. E. GOUGH, B. J. ORR, AND G. SCOLES,J. Mol. Spectrosc. 99, 143-150 (1983). 6. F. MATSUSHIMA,N. MORITA,S. KANO, AND T. SHIMIZU,J. Chem. Phys. 70,4225-4231 (1979). 7. F. MATSUSHIMA,N. MORITA,S. KANO, AND T. SHIMIZU,Appl. Phys. 24, 219-224 (1981). 8. T. SHIMIZU,F. MATSUSHIMA,AND Y. HONGUH, “Laser Spectroscopy V” (A. R. W. McKellar, T.

Oka, and B. P. Stoicheff, Eds.), p. 212, Springer-Verlag, Berlin, 1981. 9. C. DOUKETIS,T. E. GOUGH, G. SCOLES,AND H. WANG, J. Phys. Chew in press. 10. R. H. SCHWENDEMAN, private communication. Il. A. G. ADAMAND A. K. LEWIN,unpublished results. l-7. C. H. TOWNESANDA. L. SCHAWLOW,“Microwave Spectroscopy,” p. 256, McGraw-Hill, New York, 1955. 13. G. DUXBURY,unpublished Lamb dip spectra.