J . Phys. Chem. 1990, 94, 1140-1141
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AB CH31 SOR PTlON s \ P = 3 0 0
TORR
0
P=300TORR
C
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Figure 1. Absorption spectra of CH31at various experimental conditions.
Spectral resolution is 0.4 A (- 10 cm-I), node temperature was at room temperature,and the CHJ backing pressures are indicated. Nozzle pulse duration was 800 F S at -10 Hz repetition rate. The vacuum chamber was pumped with a 1200 L/s diffusion pump in A. In B and C the liquid nitrogen cold trap in front of the nozzle was filled. The differences between A and B are due to an artifact (see text). identify all their results as emerging from an experimental artifact. Our experimental apparatus for the study of absorption spectroscopy in supersonic free jets is described in detail else~here.~.’ However to exactly match the reported experimental conditions,I4 we also used a modified fuel injector as a pulsed nozzle having 1.0-mm nozzle diameter. We used a pulsed xenon lamp as a light source with a pulse rate that could be doubled as compared to the nozzle repetition rate. Thus one light pulse was synchronized with the nozzle gas pulse, and the untimed light pulse served to measure the background absorption. In normal operation the untimed absorption spectra were automatically substracted from that of the synchronized gas and light pulses. This arrangement represents a faithful reproduction of lock-in amplification that gives the difference between the in-phase and out-of-phase absorption spectra. In Figure 1A we show typical absorption spectra in the vicinity of the B Rydberg state of methyl iodide. The upper spectra was obtained by using 300 Torr of neat CH31 backing pressure as in ref 1, and it shows both the “dimer” splitting and asymmetric line shape reported.I4 It is important to note that the asymmetric “Fano” line shape reported in refs 1-4 results from a negatioe absorption and cannot be real. By merely filling the liquid nitrogen cold trap, which is placed 10 cm downstream from the nozzle, we show in Figure 1B that all these features disappear and a single broad line appears. In addition the “extracted” magnitude of absorption is increased by a factor of 2.5 upon filling the liquid nitrogen trap. These two effects can be attributed to the largely increased effective pumping speed toward CHJ and consequently ( 6 ) Amirav, A.; Horwitz, C.;Jortner, J. J . Chem. Phys. 1988,88, 3092. (7) Amirav, A. Chem. Phys. 1988, 126, 327.
to the practical elimination of the background absorption. In Figure 1C we show the absorption spectrum of CH31 obtained at a lower nozzle backing pressure of 40 Torr. This spectrum demonstrates the fact that at 300 Torr the line is much broader and the light attenuation (over 90%) is clearly out of the linear region of dependence on the molecular density. Both our experiments and those of refs 1-4 measure transmission, which represents absorption only at low light attenuation level. At these conditions it is anticipated that a further increase in the molecular density will induce an ”absorption” decrease under the experimental conditions of Donaldson et al.’-4 This reduction is due to increased contribution of out-of-phase background absorption, which has a negative sign and is more pronounced for the strongest absorption features of the monomer. Similar results were obtained by using our pulsed slit nozzle, and the artifacts could also be eliminated by the use of the timed light pulses alone (no light-off correction factor). Our rationalization of Donaldson et al. results are as follows: (a) The “dimer” formation is not due to a new line but represents a line splitting to both the blue and red sides. This is due to saturation in the in-phase light attenuation and the negative sign of the out-of-phase contribution of their lock-in amplifier output. (b) The asymmetric interferences are due to negative absorption contributions of the out-of-phase signal and are not real. It is regretful that Donaldson et al. never showed the zero absorption position. The red side negative absorption emerges from both red-shifted vibrational sequence bands, which are more pronounced in the hotter background, as well as from a small pressure-induced blue shift of the Rydberg transition* which can also be detected in Figure 1. (c) The new l i n e ~ l -that ~ appear are not new but are weak existing lines, and their height ratio to the umbrella overtone is unchanged with the nozzle backing pressure. They are more pronounced on a linear scale in comparison with the B Rydberg origin due to saturation effects in the later transition. This conclusion is also consistent with the data analysis of Raman scattering from CH31,g.10which are contrasted with the work of Donaldson et al. According to our finding, the spectral location and magnitude of (CH31)2is still an open question. We believe that it exists in the form of a smooth broad and blue-shifted absorption. Finally we note that the B Rydberg state of methyl iodide has a gas-phase molar extinction coefficient of 50000.” Any speculation about its interference with continuous absorption should be ruled out as it requires that the integrated oscillator strength of that much broader continuum will be above 1. Acknowledgment. This research was supported by the Fund for Basic Research of the Israel Academy of Sciences. (8) Messing, I.; Raz, B.; Jortner, J. Chem. Phys. 1977, 55, 25. (9) Wang, P. G.;Ziegler, L. D. J . Chem. Phys. 1989, 90, 41 15. Ziegler, L. D.; Chung, Y. C.; Wang, P.; Zhang, Y. P. J. Chem. Phys. 1989,90,4125. (IO) Lau, K.; Person, M. D.; Chou, T.; Butler, L. J. J . Chem. Phys. 1988, 89,3463. (11) Boschi, R. A,; Salahub, D. R. Mol. Phys. 1972, 24, 289.
School of Chemistry Sackler Faculty of Exact Sciences Tel Aviv University Ramat Aviv 69978 Tel Aviv, Israel
Aviv Amirav* Abraham Penner
Received: June I , I989
Reply to Comments on “Rydberg State Dynamics of Methyl Iodide Dimers and Clusters Revisited” Sir: This is in response to the comment by Amirav and Pennerl in which it is claimed that our observations of dimer-induced
0022-36S4/90/2094-1140%02.50/00 1990 American Chemical Society
Comments
The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7741
changes in the spectra of the Rydberg states of methyl i ~ d i d e ~ - ~ are experimental artifacts, due either to optical saturation or to the use of lock-in detection. Professor Amirav is correct in pointing out that there are many possible sources of error, even in a conceptually simple experiment. However, we have performed extensive tests to eliminate the possibility of such artifacts contributing to our data. The results of these tests clearly show that our published result^^-^ are quite free of artifacts and represent true cluster-induced effects. As such, the model we have developed2-’ to explain these observations is still the only one that is consistent with all the data. In their comment,’ Amirav and Penner seem to be confused on two points. The first has to do with the different effects observed in the dimer-induced spectrum vs the spectra of highly clustered methyl iodide. It is only in the latter that we observe the negative-going features to the red of the intense peaks. The dimer spectrum shows a new band in the origin region but no “Fano line shape”. The second misconception is that the negative going features represent negative absorption. As we make clear in our published ~ o r kthere ~ , ~is a strong continuum that grows in with increasing backing pressure; the negative-going features represent a decrease in the continuum absorption. Such a continuum is expected in the high cluster limit, on the basis of the known matrix spectra8 of methyl iodide. As we have ~ t a t e d we , ~ observe no difference in the spectra whether we use lock-in detection or a boxcar integrator; the same negative-going features are observed by measuring directly the PMT output. Furthermore, these same features are seen in spectra of methyl iodide embedded in large rare-gas clusters! Together, these observations indicate that the absorption spectra are not plagued by an artifact, either of the detection method or of interference due to room-temperature background absorption. They may be due to Rydberg-continuum interference, as we initially suggested,24 or to Rayleigh scattering by the c l ~ s t e r s . ~ Our data do not support the allegation that optical saturation is responsible for the observed spectroscopic effects. In particular, we wish to stress the following points: The spectra we r e p ~ r t e d were ~ - ~ measured under conditions such that the light attenuation is typically 530%. Under these conditions, the relative intensities of spectral features are within 5% of their relative absorbances.I0 We have investigated extensively the dependence of spectral intensities and vibronic line shapes on light intensity and resolution since our first absorption studies of jet-cooled molecules.’l Many of these tests, which were subse-
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quently used to ensure proper operating conditions, were performed by using the ‘B2(1Z+2)transition of CS2,10J’which is 20 times more intense than the Rydberg transition in CHJ and should therefore more easily show saturation effects. Specifically, in the case of methyl iodide, no splittings were observed as a function of light intensity and slit widths in tests run to explore this possibility. In one of the tests we performed, we decreased the light intensity falling on a dilute molecular jet containing only monomers, so much as to obtain a 90-100% attenuation of the light at the peak of the origin transition. No splitting or other artifacts were observed. The new feature we observe and assign to the dimer2-“ is not induced by changes in the light intensity or slit width; it appears only under expansion conditions that promote dimer formation. This “splitting” is not symmetric about the monomer origin, but, as reported in this journal3 and elsewhere,” represents the growth of a new feature, IO cm-’ to the red of the monomer origin band, with a relative intensity that depends upon backing pressure. The identical expansion conditions give rise to the onset of dimer signal in a complementary REMPI study.6 Under the most extreme operating conditions used for the dimer studies, the maximum contribution of room-temperature background gas to the signal is ~ 1 5 % . This is in contrast to the operating conditions of the Amirav and Pennerl experiment. There, very long path lengths of neat methyl iodide (a strong absorber!) are used, and consequently background absorption is far more important than in our studies. In our experiments on other samples, we see no splittings even when stronger absorbers are involved. Examples are the 200-nm band of CS2I0and also the Rydberg state of acetone.I2 In this latter example5J2we do report cluster-induced spectral changes yet observe no “splittings” or “Fano line shapes”. In a 1-mm path length cell, we have measured pressure-dependent peak extinction coefficients for the vibronic modes we report as being enhanced through dimer Specifically, at pressures above 50 Torr, we observe an increase in the peak extinction coefficient with increasing pressure, as we expect for features that are enhanced by dimer formation. We believe that these points, many of which have been published in papers refered to by Amirav and Penner,’ effectively show that our observations of cluster-induced effects in the Rydberg spectra of methyl iodide are real. Finally, we note that in our experience, the efficiency of methyl iodide dimer formation is strongly dependent on the exact nozzle configuration used (Le., pulse length, shape of orifice, etc.). This might explain Amirav and Penner’s failure to reproduce our results.
(1) Amirav, A.; Penner, A. J . Phys. Chem., preceding comment in this
issue. ( 2 ) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Chem. Phys. 1987, 87, 2522. (3) Donaldson, D. J.; Vaida, V.; Naaman, R. J . Phys. Chem. 1988, 92, 1204. (4) Donaldson, D. J.; Sapets, S.; Vaida, V.; Naaman, R. In Lurge Finire Systems; Jortner, J., et al., Eds.; Reidel: Dordrecht, The Netherlands, 1987. (5) Vaida, V.; Donaldson, D. J.; Sapers, S.P.; Naaman, R.; Child, M. S. J . Phys. Chem. 1989, 93, 513. (6) (a) Sapers, S. P.; Vaida, V.; Naaman, R. J . Chem. Phys. 1988, 88, 3638. (b) Sapers, S. P.; Vaida, V., manuscript in preparation. (7) Donaldson, D. J.; Child, M.S.; Vaida, V. J . Chem. Phys. 1988, 88, 7410. (8) (a) Gedanken, A.; Karsch, Z.; Raz, B.; Jortner, J. Chem. Phys. Lett. 1973,20, 163. (b) Gedanken, A.; Raz, B.;Jortner, J. J . Chem. Phys. 1973, 58, 1178. (9) Similar features have been observed in the infrared spectrum of (CO,), by Gough and co-workers (personal communication) and attributed to Mie
scattering. (IO) hemley, R. J.; Leopold. D. G.;Rocbber, J. L.;Vaida, V. J. Chem. Phys. 1983, 79, 5219.
( 1 1 ) Vaida, V. Am. Chem. Res. 1986, 19, 114, and references therein.
(12) Donaldson, D. J.; Gaines, G. A.; Vaida, V. J. Phys. Chem. 1988.92, 2166.
Department of Chemistry and Biochemistry University of Colorado Boulder. Colorado 80309-021 5 Department of Chemistry University of Toronto Toronto, Ontario, Canada M5S 1 A1 Department of Isotope Research Weizmann Institute of Science Rehouot, Israel
V. Vaida*
D. J. Donaldson*
R. Naaman*
Received: August 15. 1989; I n Final Form: June 27, 1990
