J. Phys. Chem. 1984,88, 6685-6692
6685
Photodissociation Dynamics of Nozzle-Cooled ICN W. J. Marinelli,+N. Sivakumar, and P. L. Houston* Department of Chemistry, Cornell University, Ithaca, New York I4853 (Received: June 12, 1984)
CN(v-0) rotational distributions as a function of photolysis wavelength have been probed by laser-induced fluorescence following photodissociation of ICN prepared in very low rotational levels by expansion through a supersonic jet. In most cases, the rotational distributions can be described by a superposition of low-temperature and high-temperature Boltzmann distributions. A semiclassical model has been used to interpret the results. Potential surfaces are proposed which reproduce both the deconvoluted ICN absorption spectrum and the major features of the variation in average rotational energy disposal as a function of photolysis energy. Minor discrepancies between the theory and experiment are likely to be caused by nonadiabatic transitions between dissociative surfaces.
I. Introduction The photodissociationof ICN in the A continuum has long been an object of study and controversy.'-13 Although an excellent discussion of the experimental work prior to 1979 has been provided by Morse et al.: a brief summary will be given here in order to highlight recent advances and emphasize outstanding problems. Ling and Wilson were the first to use a time-of-flight spectrometer to measure the recoil velocity distribution of the fragments of ICN dissociation at 266 nm. The two peaks observed were attributed either to the production of CN(X,u=O) with I* I(2Pljz)and I I(2P3iz)or to the production of I with CN(X) and CN(A).2 More recent work using laser-induced fluorescence to monitor the CN(X) vibrational and rotational distribution has shown that this radical is produced primarily in its ground electronic state and lowest vibrational l e ~ e l . ~Furthermore, ,~ two studies have demonstrated that photolysis of ICN at 266 nm produces I*, roughly in the yield which would be expected from identification of the Ling and Wilson peaks with I and I*.',* Thus, it appears that dissociation of ICN in the A continuum can lead either to CN(X) + I or to CN(X) + I*. At least three electronic states of ICN are involved in the dissociation.8 On both the high- and low-energy sides of the continuum, dissociation leads predominantly to CN(X) + I, while in the center of the continuum near 266 nm, dissociation favors the CN(X) + I* channel. At least one of these three states must be bent, because the data of Ling and Wilson indicate that both I* and I are produced via parallel transitions* whereas a linear state of ICN which also correlates with CN(X) + I cannot be reached by a parallel transition.6 Since a bent ICN intermediate is likely to give rise to rotationally excited C N fragments, it was one purpose of the current work to see if a detailed examination of the nascent rotational distribution could be used to determine the geometry and ordering of the electronic states involved in the dissociation. Two new experimental techniques were needed to perform this study. First, in order to study the different electronic states, we needed to perform the dissociation with variable-frequency laser sources. Second, in order to ensure that the C N rotational distribution was characteristic of the dissociation process and not simply a reflection of the rotational angular momentum of the ICN parent compound, we needed to perform the dissociation with ICN cooled to a few degrees kelvin by expansion through a nozzle jet, Details of these techniques are given in section 11. Since starting our studies, we have learned of several other groups which have worked along the same lines. Fisher et a1.I1 have used variable dissociation wavelengths and probed the C N rotational distributions, but the ICN was not cooled. BaronavskiI2 and Nadler et al.I3 have performed experiments very similar to ours. It appears from preliminary reports that all four groups are in substantial agreement on the nature of the experimental results. In our interpretation, these results suggest that the ICN
electronic state leading to C N + I* is linear, while the two states leading to C N I are bent. The upper of the two C N I surfaces crosses the surface leading to C N + I*.
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11. Experimental Section
Laser-induced fluorescence of CN on the B22+-X22+transition has been employed extensively by previous workers to determine the nascent rotational and vibrational distributions following photodissociation of cyanogen-containingm o l e ~ u l e The s ~ ~static ~~~~ cell and molecular beam studies described here both employed this technique. Several different laser sources were used to provide a variety of photolysis wavelengths in order to access the various electronic states that comprise the ICN A band. A schematic diagram of the pulsed molecular beam apparatus, which varies in only mechanical ways from the static cell apparatus, is shown in Figure 1. The experiments are described in more detail below. Static Cell Experiments. Photolysis of ICN in a static cell was conducted at 266, 280, and 290 nm. The photolysis cell, constructed of Pyrex and externally blackened, was 65 cm long, 3.8 cm in diameter, and equipped with Brewster-angle windows and Wood's horns at each end to reduce scattered light from window reflections. Fluorescence was viewed at right angles to the counter-propagating photolysis and probe laser beams through a Pyrex window centrally located along the cell axis. The cell was connected to a standard glass vacuum manifold and was evacuated to a pressure below torr before the start of each experiment. ICN was introduced into the cell through a separate port, and its pressure was measured with a capacitance manometer (MKS-221-HS-10) connected directly to the cell. An iris and collimating lens (100-cm focal length), placed at the entrance window of the cell, limited the photolysis beam cross section to 4-5 mm in diameter, which matched the diameter of the probe beam. At the typical dissociation laser fluences employed (2 mJ/cm2 at 266 nm), less than 1% of the ICN was photolyzed over the course of one experiment. ICN pressures of typically 8-13 mtorr were used, and nascent C N product state distributions were recorded at delays between 25 and 100 ns following photolysis. The collision probability for a C N fragment during these delay times is less than 2 X Donovan, R. J.; Konstantatos, J. J . Photochem. 1972, 1 , 75. Ling, J. H.; Wilson, K. R. J . Chem. Phys. 1975, 63, 101. Sabety-Dzvonik, M. J.; Cody, R. J. J . Chem. Phys. 1977, 66, 125. Baronavski, A. P.; McDonald, J. R. Chem. Phys. Lett. 1977, 45, 172. Beswick, J. A,; Jortner, J. Chem. Phys. 1977, 24, 1. Morse, M. D.: Freed, K. F.; Band, Y. B. J . Chem. Phys. 1979, 70, 3620.
(7) Arnimoto, S. T.; Wiesenfeld, J. R.; Young, R. H. Chem. Phys. Lett. 1979, 65, 402. (8) Pitts, W. M.; Baronavski, A. P. Chem. Phys. Lett. 1980, 71, 395. (9) Baronavski, A. P. Chem. Phys. 1982, 66, 217. (10) Kreiger, W.; Hager, J.; Pfab, J. Chem. Phys. Lett. 1982, 85, 69. (1 1) Fisher, W. H.; Carrington, T.; Filseth, S.V.; Sadowski, C. M.; Dugan, C. H.Chem. Phys. 1983, 82, 443.
+Currentaddress: Physical Sciences Inc., Research Park, P.O. Box 3100, Andover, MA 01810.
0022-3654/84/2088-6685$01.50/0
(12) Baronavski, A. P., private communication. (13) Nadler, I.; Reisler, H.; Wittig, C. Chem. Phys. Lett. 1984, 103, 451.
0 1984 American Chemical Society
6686 The Journal of Physical Chemistry, Vol. 88, No. 26, 1984
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Marinelli et al.
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i“”’ i l Figure 1. Schematic diagram of the experimental apparatus.
Pulsed Molecular Beam System. The pulsed molecular beam system employed is of a conventional design, consisting of counter-propagating probe and photolysis beams with fluorescence viewed perpendicular to both the molecular and laser beam axes. The molecular beam nozzle, which produces pulses of 15O-~sfwhm duration at 10 pps, is of the type described by Adams, Rockney, Morrison, and Grant.I4 ICN (Fluka), seeded 1:1500 in He at 2-atm stagnation pressure, was photolyzed 1.5 cm from the nozzle. Under similar expansion conditions, a 10% NO/He mixture was found to cool N O to a rotational temperature of 3.2 K. While rotational cooling is not as efficient for ICN as it is for NO, the increased dilution factor employed in these experiments may compensate for the decreased efficiency. At 3 K the rotational level of ICN with the largest population is 3, as opposed to 32 at 300 K. Lasers. An excimer pumped dye laser (Lambda Physik EMGIOl/FL2002) using BBQ laser dye was used to pump directly the CN(0-0) transition. The dye laser amplifier was removed for all experiments, resulting in typical pulse energies of less than 150 pJ. Due to the large oscillator strength of the CN(B-X) transition, the probe laser intensity was further reduced by a series of neutral density filters to an intensity of -10 pJ in the static cell experiments, where reduced signal to noise could be tolerated. Possible effects of saturating the C N transition with the probe laser will be discussed in a later section. Photodissociation of ICN at 266 nm was accomplished using the fourth harmonic of a Nd:YAG laser (Quanta Ray DCR l/lA). Other photolysis wavelengths employed were 248 nm (KrF excimer, Lambda Physik EMG 101) and 235,280, and 290 nm (Nd:YAG pumped dye laser with harmonic and sum frequency generation, Quanta Ray DCR 2A/PDL-l/WEX-l). Defection System. Fluorescence from the CN(B-X) transition was monitored by a photomultiplier (Hammamatsu R928) equipped with color filters (Corning 7-54, Schott WG320) for rejection of scattered photolysis light. The PMT signal was recorded with a boxcar signal averager (PAR 162) and fed into a computerized data acquisition system (LSl 11/02) which also scanned the probe laser. 111. Results and Analysis Typical spectra of the CN(O-O) transition following photolysis of ICN in a static cell at 290 nm and in the molecular beam at 266 nm are shown in Figures 2 and 3, respectively. Similar spectra were obtained at 280 and 266 nm in the static cell and at 290, 280, 248, and 235 nm in the molecular beam. Each of these spectra comprises a single band system with an origin at 387.6 nm. The system consists of single P and R branches; the P branch forms a band head at 388.3 nm. Due to the large amount of rotational excitation present, the P branch is highly congested and only the R branch may be used for rotational analysis. The origin of the (1-1) band, which is overlapped by the (0-0) band, occurs at 386.4 nm with the P-branch band head falling under R(5) of (14)
Adams, T. E.; Rochey, B. H.; Morrison, J. S.; Grant, E. R.Rev. Sci.
Instrum. 1981, 52, 1649.
3810
389 C
3853
\A’cde1eng:t- (- rr) Figure 2. Typical experimental spectrum for photolysis of ICN in a static cell. The dissociation wavelength was 290 nm.
e
1 1
U
c .-c” L?
U
0
38‘ c
339 c
3853
iiclveleflgtr (7 Figure 3. Typical experimental spectrum for photolysis of ICN in the molecular beam. The dissociation wavelength was 266 nm.
the (0-0) transition. Previous estimates of populations in ut’ = 1 of C N following photodissociation of ICN are less than 1% at 266 nm and less than 6% at 299.1 nm$~10representing a negligible contribution to the total observed intensity of the band and a small contribution to the intensity of R(5) and R(6) of the (0-0) transition. Our results support the previous conclusions, and therefore only populations in u” = 0 will be considered here. Possible Saturation Effects. Previous results for the photolysis of ICN at 266 nm under static cell conditions showed a rotational distribution for the (0-0) transition that could be parameterized by the sum of three Boltzmann-like functions characterized by “cold“ ( T < 100 K), “intermediate” ( T = 500 K), and “hot” ( T > 5000 K) rotational temperature^.^^^-'^ It has been suggested that these peculiar rotational distributions are an experimental artifact resulting from saturation of the excitation transition. Since saturation may affect the apparent rotational distribution we observe, it is important to know under what conditions saturation occurs and how it is manifested in the excitation spectrum. Under the conditions employed in these experiments, fluorescence is only observed following termination of the excitation pulse. Thus, only spontaneous emission is observed from the excited level, and the emission rate is proportional to the population of that level. Saturation of the excited transition affects the observed rotational distributions by limiting the population of the excited level and hence the observed emission rate. The onset of saturation for a transition occurs when the rate of stimulated emission from the level being excited becomes comparable to the rate of spontaneous emission from that level. We can use a simple two-level system to model how saturation of a transition affects the population of the upper level in our experiments. During the excitation pulse the rate of change in the population of the upper level is given by d[n’(t)]/dt = n”pB”- n’(A
+ pB’)
(1)
where n’and n”are the populations of the upper and lower levels, B’land B’are the Einstein coefficients for absorption and stimulated emission, A is the Einstein coefficient for spontaneous
Photodissociation Dynamics of ICN
The Journal of Physical Chemistry, Vol. 88, No. 26, 1984 6687
emission, and p is the energy density of the excitation field. Given the constraint that the total number of particles, no = n’ n”, must be conserved and that the excitation pulse is a square wave with duration r, the population of the upper level at pulse termination is given by
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In the limit of weak fields (when p[B’+ B”J
