IR diode laser spectroscopy in molecular beams

lnlrared Phys. gol. 25. No. 1/2, pp. 215 218, 1985 Printed in Greal Britain. All rights reserved

0020 0891/85 $3.00 + 0.00 Copyright ,,~ 1985 Pergamon Press Ltd

IR D I O D E LASER S P E C T R O S C O P Y IN MOLECULAR BEAMS P. B. DAVIES and A. J. MORTON-JoNES Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEP, England

(Received 27 July 1984) Abstract

A molecular beam spectrometer for recording cold-jet spectra is described. Modulation of the

molecular beam is provided by a mechanical chopper. Someresults on CO, C3H~ and CF3CI are presented.

INTRODUCTION In the past, the analysis of rovibrational spectra of polyatomic molecules in the IR has been hindered by the lack of spectrometer resolution. With the advent of lasers (especially diode lasers) and the recent development of F T I R instruments, Doppler-limited resolution can now be achieved. However, even at this resolution, the spectra of some molecules remain intractable due to the vast number of allowed transitions. The remarkable cooling properties of supersonic expansions are now being used for the first time to simplify these spectra, It- 3~ by providing hot-band depletion and simplification of rotational structure. The method is still in its infancy, and not without its problems, the principal one being lack of sensitivity. However, recent results have shown that the method has great potential as a very useful spectroscopic tool. S U P E R S O N I C BEAMS, The properties of molecular beams formed by the expansion of gas into a vacuum through a small pinhole or nozzle have been studied in detail for many years. If the length of the nozzle is large compared to the mean-free-path for collisions, then a supersonic expansion occurs. Many collisions take place in the nozzle resulting in a narrowed velocity distribution on exit. This narrowing implies a cooling of the translational motion, even though the actual velocities of molecules may be above mean velocity at room temperature. U p o n exit from the nozzle, binary collisions equilibrate the internal and translational temperatures with consequent rotational and vibrational relaxation. It is generally true that R - T relaxation crosssections are comparable to the gas-kinetic cross-section, whereas the V -T relaxation cross-section is smaller so that Vt. . . . ~ Zro! < Tvib'

Mach numbers in the expansion can reach large values, mainly due to the low local velocity of sound (~ T }). EXPERIMENTAL Figure 1 shows schematically the arrangement of the molecular beam spectrometer. Gas expands through a 50 or 100 #m nozzle into an evacuated chamber pumped with a pumping speed of 15001/s. Backing pressures of 1-4 atm are typically used. Laser radiation is supplied by diode lasers (Spectra Physics) housed in a CTI Cryogenics coldhead. The laser beam crosses the molecular beam up to eight times by means of a system of White-type mirrors. The radiation then passes through an f / 4 m o n o c h r o m a t o r (Edinburgh Instruments Ltd) and onto an HgCdTe detector. Relative calibration is provided by a 1" Ge etalon. A rotating sector positioned close to the nozzle modulates the beam at around 2 kHz and provides a reference for phase-sensitive detection. This modulation technique has an increased sensitivity over iNv 25:1/2-o

215

216

P.B. DAVIES and A. J. MORION-JONES

.__[

GAS LINES

PSD

LOW-PRESSURE ] - ~ =DIODE LASER COLDHEAD

[ ~ . ~ MONO~T~HROM ATOR

c°ggfgT[

t LASER

X

IRECORDER]

Fig. 1. Schematic diagram of the molecular beam laser spectrometer.

laser source modulation, gives a flat baseline and eliminates absorption due to background gas in the chamber. The main source of noise is the 3 Hz vibration of the coldhead refrigeration system and this can be minimized by moving to higher modulation frequencies. R E S U L T S AND D I S C U S S I O N To verify that substantial cooling takes place in the molecular beam, an intensity analysis was carried out on CO transitions in order to determine the rotational temperature. The absorption intensity, I, should vary as

I/I o = a(d + l)e -~'*t~+ l l,'kr where Io = transmission intensity, J = rotational quantum number and a = constant. A measure of I/Io in the beam was obtained for the transitions R(0) to R(7). Nozzle diameter was 50/2m and the backing pressure was 50psig (unseeded beam). Figure 2 shows a plot of ln{NJ/[No(J + 1)]I vs J(J + 1). A Boltzmann distribution should give a straight line, gradient - B/kT. The departure from the thermal distribution is such that the high J-values are overpopulated, presumably because the R Trelaxation cross-section is smaller for high J. Bassi et al) 41 observed similar effects in their expansion of CO. The "pair temperatures", T OJ, varied from T °1 = 21.2 K to T oo = 34.9 K, confirming that considerable cooling takes place. To illustrate the success of this technique in simplifying IR spectra, the spectra of two molecules, C3H6 and CF3C1, were investigated in detail. The v~l(e') band of cyclopropane centred at 868.5cm ~ has been studied several times under medium resolution. ~> vl Interest in this band arises in part from a perturbation known as ( + 2, + 2) Iresonance. We have recorded many molecular beam spectra of this band, mainly Q-branches. In

IR diode laser spectroscopy in molecular beams

217

-20 -25 -3,0 -3.5

5

-4.5 -50 -5.5 -6,0 I 10

I 20 J(d+ 1)

I 30

I 40

Fig. 2. Plot of ln{NJ/[No(J + 1 )]} vs J(J + |) to determine the rotational temperature of a supersonic expansion of CO.

most cases, both the structure and origin of the Q-branch were obscured by high J-components of other Q-branches in the room temperature cell spectrum, whereas the structure became greatly simplified in the molecular beam spectrum. Figure 3 shows a molecular beam spectrum of the RQ0-branch of the vll perpendicular band. The spectrum clearly shows:

(i)

1-resonance effects leading to divergence of J-components, in contrast to the other RQ-branches, where they are compressed together; (ii) the predicted 2:1 alternation in intensity for odd:even J due to nuclear spin statistics. A partial analysis of the band has been completed and the values for molecular parameters in the v11 = 1 state are given in Ref. (3). More recently we have obtained spectra on the vx(al) band of CF3C1 at ~ 1108 cm- 1. The band is complicated by the natural abundance of both C1 isotopes and also by the presence of several hot bands. In an 8:1 expansion mixture of Ar:CF3C1 the hot-band transitions were greatly reduced in intensity, thereby enabling facile assignment of the P- and R-branches of the fundamentals of both isotopomers. Furthermore, the central Q-branch shows no feature corresponding to the beginning of the CF337C1 °Q-branch, whereas this is apparent in the molecular beam spectra. In summary, the spectrometer has been shown to produce spectra which are greatly simplified due to low internal temperatures. We have found that the most dramatic results are obtained if the molecule has either many hot bands (in which case a dilution with Ar is fairly essential to reduce the vibrational temperature) or the rotational constants are fairly large (e.g. C3H 6 with B = 0.67cm- 1, A = 0 . 4 2 c m 1).

R

Qo

1 7

I

I

I

I

I

I

6

5

4

3

2

1

d

Fig. 3. Molecular beam spectrum of the RQo-branch of the v11 fundamental band of cyclopropane. The etalon-free spectral range is 0.0482 + 0.001 c m - i.

218

P.B. DAVIES and A. J. MORTON-JONES REFERENCES

1. 2. 3. 4. 5. 6. 7.

Mizugai Y., Kuze H., Jones H. and Takami M.. Appl. Phys. B32, 43 (1983). Takami M. and Kuze H.. J. chem. Phys. 80, 5994 (1984). Davies P. B. and Morton-Jones A. J., ('hem. Phys. Lett. 107, 27 11984). Bassi D., Boschetti A., Marchetti S., Scoles G. and Zen M., J. chem. Phys. 74, 2221 (1981). Cartwright G. J. and Mills 1. M., J. molec. Spectrosc. 34, 415 (1970). Duncan J. L., J. molec. Spectrosc. 25, 451 (1968). Masri F. N. and Blass W. E., J. molec. Spectrosc. 39, 21 (1971).