1389
J . Phys. Chem. 1993,97, 1389-1399
State-to-State Collisional Energy Transfer in Electronically Excited SiCl Radicals Scott Singleton+and Kenneth C. McKendrick’ Department of Chemistry, The University of Edinburgh, Edinburgh EH9 355, U.K. Received: August 12, 1992; In Final Form: November 2, 1992
The collisional quenching of selected vibrational levels of electronically excited Sic1 B’ZA radicals by the series of nonpolar molecules He, Ar, H2, N2, C02, CH4, and CF4 was investigated experimentally. The results for He and Ar agreed well with previous independent measurements. Rate constants (and cross sections) for total removal of Sic1 B12A v’ = 0 and 1 are relatively large for all collision partners and quite well correlated with the attractive nature of the intermolecular interaction. Rate constants and cross sections were measured for transfer to the near-degenerate B2Z+electronic state. A substantial, but variable, fraction of the total removal proceeds via this channel for all quenchers. Nascent vibrational distributions over the V I = 0, 1, and 2 vibrational levels of the B2Z+ state were determined by exploiting the relatively much shorter radiative lifetime of this electronic state. N o obvious, single molecular property could be identified to explain these vibrational distributions. There is, however, a trend toward removal of more energy by polyatomic molecules. The cross sections for the single, near-resonant state-to-state process B12A v’ = 0 to BZZ+u’ = 2 were found to be well-correlated with attractive intermolecular forces. Selected higher resolution measurements confirmed the lack of any significant vibrational relaxation within the B’2A state competitive with collisional transfer to the B2Z+state and revealed no evidence for preferred interstate transfer through specific -gatewayn rotational levels.
I. Introduction
Energy transfer during collisionsbetween molecules is a process of wide-ranging practical and fundamental significance. In a general way, it is responsible for the establishment of thermal equilibrium in a great variety of systems, such as those in which exothermic chemical reactions selectively release energy to the products. It governs the concentrations of excited species in high energy environmentssuch as flames, plasmas, or laser gain media. It also affects the efficiency with which such molecules can be monitored spectroscopically,for example by observing fluorescence from those either spontaneously formed in excited states or more actively pumped there by some form of probe radiation. More specifically, this paper is concerned with collisions of Sic1molecules state-specifically prepared in the lower vibrational levels of the B’2A electronic state. In particular, we attempt to discover the fate of Sic1 molecules collisionally removed from this state and to understand how the behavior is related to the identity of the collision partner. There have been many measurements of the total rates of collisional removal of particular electronically excited states of small molecules. A much smaller subset of investigations has probed the state-testate propensities for transfer on particular product channels. Those which have been studied most directly include even-electron neutral molecules, e.g.1-5 Na2 B’n, (2)lZg+, 0 2 blZg+ alA,, and N2 B3ng B’3Z,- and W3A,; odd-electron radicals, e.g6-ll CN A 2 n X2Z+and B W , NH c l n A3n,and OH A W X2n;and ions, e.g.lZ-l8N2+ A2n, X2Zg+ and CO+ A 2 n X2Z+. Various models been advanced in an attempt to rationalize the influenceof different physical factors on the interelectronic state collisional transfer. “Gateway” levels, through which transfer is channeled preferentially, have been deduced to be important in cases such as certain vibrational level^^^^^^ of NS BZn,the BaO AIE+state,21and the CS A I n state.22 The levels involved are spectroscopically perturbed even in the absence of a collision partner. However, in at least one system, CN A211 v’= 10 B2Z+ v‘ = 0, originally thought to be governed by such a
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* Author to whom correspondenceshould be addressed.
Current address: Port Sunlight Laboratory, Unilever Research, Merseyside L63 3JW, U.K.
0022-3654/93/2091-1389%04.00/0
m e c h a n i ~ m , ~it~has - ~ ~been inferred from more refined experi m e n t ~that ~ the collisional transfer is equally efficient for both perturbed and unperturbed rotational levels. This is alsodefinitely true of collisional coupling of other vibrational levels of CN A2n to the X2Z+state.6-s Gateway levels of this type certainly cannot be involved when perturbations between the collisionally coupled states are symmetry forbidden, as in, among others, the Na2, N2, and Nz+ examples a b o ~ e . I ~ ~ -Perhaps ~ J ~ - ~one ~ of the more surprising contradictory recent results,Is therefore, is that for the v’ = 0 level of CO+ A2n, which is isoelectronic with CN A 2 n and N2+ A211,, collisions with He only efficiently transfer population to the X2Z+state through specific perturbed rotational levels. Energy gap scaling laws, incorporating both Franck-Condon overlap between the initial and final vibronic states and an exponential energy gap term appropriate to repulsive26 or attractivez7intermolecular forces, have been found to be of very questionable utility in explaining experimental results for the CN, N2+,CO+series.68J2-’8 It is widely observed in these studies that collision-induced electronic state changes are accompanied by small changes in rotational quantum number, regardless of the size of the vibrcjnic energy gap involved. The relative cross sections for channels with widely differing energy gaps and Franck-Condon factors are not quantitatively predicted by the scaling laws. More rigorous quantum calculationsZ8for the particular collision-induced process 211 ZZhave cast further doubt on the role of the Franck-Condon factor. Katayama and Dentamaro have suggestedI8 that their results for CO+ may indicate that a very small Franck-Condon factor enhances the importance of gateway levels. Clearly, no universal, simple, predictive generalization has yet been discovered for electronic energy transfer in small molecules, and it remains an area of active research. The only directly relevant previous work on energy transfer in Sic1 is that of J e f f r i e ~ ,who ~ ~ ,was ~ ~ the first to observe efficient collisional quenching of Sic1 BQA by the rare gases He, Ne, and Ar. Transfer to the near-degenerate B22+state was found to be a major channel. The propensities for production of different B state vibrational levels were remarkably sensitive to the identity of the collider, even for this simple series. In the present work we extend these measurements to include the wider range of
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0 1993 American Chemical Society
1390 The Journal of Physical Chemistry, Vol. 97, No. 7, 199’3
i ’! I
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Singleton and McKendrick example, the conclusion of Bruno et al.j7 that two independent processes must lead to the formation of Sic1 B’ZA in SiC14/H2/ Ar plasmas. This was reasonably inferred from the observation of different ratios of B’ZA-XZII to B2Z+-X211emission intensities in different gas mixtures. In fact, with the benefit of the new results presented in this paper, this particular aspect of Bruno et al.’s observation^^^ may be more straightforwardly re-interpreted as a consequence of differences in the way in which H2 and Ar collisionally transfer Sic1from the B’2A state to the B W state. 11. Experimental Section
The apparatus has been described in outline previously.31 Electronic ground state Sic1 radicals were created in a quartzglass inlet of a discharge-flow system by microwave discharge in a dilute (typically 1:lOOO) mixture of Sic&in Ar carrier gas. The 34 1.80 2.00 2.20 discharge products passed into the main axis of the system,where INTERNUCLEAR SEPARATION/A they were mixed with a second independent,and normally larger, Figure 1. RKR potential energy curves for the Sic1states of most interest flow of quenching gas. Total pressures w e p generally in the in this work. Rotationless energy gaps are indicated in units of cm-I. range of 1-6 Torr, as measured close to the fluorescence observation zone by a capacitance manometer (MKS Baratron (nonpolar) quenchers He, Ar, Hz, N2, C02, CH4, and CF4. In 222BA, 0-100 Torr). Flow was maintained by a high throughput addition, we present some complementary partially rotationallymechanical pump (Alcatel 2063, nominal 1100 L min-I). For resolved measurements. We attempt to rationalize the observed reduction of heterogeneous wall losses of radicals, the internal behavior, where possible, in terms of the properties of the surfaces of the quartz-glass discharge inlet were pretreated with quenching molecules. phosphoric acid, and the main axis of the Pyrex flow tube was In a previous joint p~blication,~’ we have presented the internally coated with halocarbon wax. spectroscopicinformationon Sic1necessary for this and J e f f r i e ~ ’ ~ ~ Production of Sic1 was maximized at relatively low discharge energy-transfer studies. The region of the intersecting B’2A- and powers (typically 15 W in our apparatus), presumably due to B*Z+-state potential energy curves of most interest in these subsequent fragmentation to Si and C1 at higher powers. It was investigationsis shown in Figure 1. The curves were constructed found also that Ar was a considerably more efficient carrier gas by the RKR p r o c e d ~ r e , jusing ~ - ~ ~published molecular constants for Sic1production than, for example, He. This perhaps suggests as described in our earlier work.” Energy gaps (in cm-’ units) that Ar metastable species may be involved in the mechanism for between the relevant rotationless vibronic levels are included in Sic1 formation. Therefore, Ar was used as the carrier of SiC14 the figure. through the discharge in all of these experiments. As will become The basis of these experiments is selective laser excitation of apparent below, Ar transpired (rather fortuitously) not to be ground-state Sic1 molecules to either u’= 0 or 1 of the B’2A state particularly efficient in the collision transfer of Sic1 from the B‘ in the presenceof the quenching gas. Collisions transfer a certain to B electronic states and to deposit most of the population in a proportion of the molecules to the lower-lying u’ = 0, 1, and 2 single B-state vibrational level. Consequently,the component of vibrational levels of the B2Z+state. The populations in both the AInecessarily present in all these experiments did not substantially initially prepared B’2A-state level and the collisionally-produced complicate the observation of quenching by other gases. B2Z+-statelevels are monitored by dispersing the fluorescence Fluorescencewas excited from the Sic1radicals by the pulsed on the respective B’2A-X211 and B2Z+-X211transitions.31 output of a Nd:YAG laser-pumped dye laser system (Spectron This relatively simple experimental procedure successfully Laser Systems SL801, Quanta-Ray PDL-2 and WEX-1A) well yields nascent vibrational populations in the B22+state because downstream (ca. 50 cm) of the discharge and the point at which of the fortuitouslydissimilar radiative lifetimes of these electronic the gases mixed. The laser radiation was tunable in the states. The B2Z+-state lifetimejs of -10 ns is 2 orders of approximate range of 275-300 nm, overlappingvarious features magnitude shorter than that of the B’2A state.30 It isconsequently of the B-X and B’-X band systems3I of SiCl. Maximum power possible to arrange the pressure of quencher such that B’2A-state densities employed were typically 500 pJ cm-2 in a ca. 10-ns molecules have a high probability of making collisions prior to pulse. In some cases the power was attenuated when it was emission but the much shorter-lived,collisionally-produced B*Z+desirable to eliminate optical saturation effects. state molecules have effectively none. State-to-state information The emission was observed perpendicular to the laser beam can therefore be obtained for this system without requiring the and main flow axes. A series of lenses (Spectrosil B) focused the additional technical complexity of optical-ptical double resolight onto the entranceslit of a monochromator (Hilger and Watts, nance elegantly applied in some other cases.6-8J2-16J8 Monospek 1000,l m) with a photomultiplier tube (EM1 9789QB) In addition to contributing to the fundamental understanding detecting the transmitted intensity. Signals were captured by a of energy transfer in electronically excited small molecules, the boxcar integrator (Stanford Research Systems SR250) and results of this paper are directly relevant in at least one applied recorded on a conventional strip-chart recorder. Peak areas in field. It is known36-38 that Sic1 is present in certain plasmas used resultant spectra were determined using an image-analyzerdevice for the etching and deposition of silicon materials in the (Kontron IBAS). semiconductor industry. If a spectroscopic technique, such as laser-induced fluorescence (LIF), were to be used to monitor Gases were obtained from BOC, with stated purities as ground electronic state Sic1 concentrations, knowledge of the indicated: Ar (99.998%), He (99.998%), H2 (99.99%), N2 quenching behavior of the electronically excited states accessed (99.998%), C 0 2 (99.8%), CH, (99.2%), and CF4 (Electra I1 would bee~sential.~~ In a related type of experiment,observations grade). Sic14 was kindly supplied by Dr. S. Cradock, of The have been made37J8of the spontaneous emissions from electronUniversity of Edinburgh, with an original stated purity of ically excited Sic1 produced directly in such plasmas. Clearly, >99.999% and was used without further purification other than collisional quenching processes must be included in mechanisms the usual removal of dissolved air by freeze-pumpthaw cycling proposed to explain these results. We note, as an illustrative at liquid nitrogen temperatures.
The Journal of Physical Chemistry, Vol. 97,No. 7,1993 1391
SIMULATION
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_ _WAVELENCTH/nm _ Ar
excitation on this already inherently congested Q-branch transition. The population in B' u' = 0 was also enhanced over the small component directly excited to higher rotational states of B u'= 2. It can be seen from LIFexcitation ~ p e c t r athat ~ , ~branchts ~ of the E X (2,O) transition underlie the B'-X (0,O)Q1 head. These spectra were recorded at lower laser powers where the relative intensities reflect in part the lower B'-X electronic transition probability. This factor is substantially removed if the excitation is optically saturated. Consequently, an effectively negligible proportionof the time-integrated emission we observed when exciting the B'-X (0,O)QIhead was from directly populated B u' = 2. As a further check of this point, we recorded a considerable number of emission spectra where the detection gate was delayed by 100 ns from the excitation pulse. Neither the ratio of B to B' populations nor the B-state vibrational distributions were measurably different from those deduced from emission spectra collected over all times following excitation. Apparent in Figures Za(i),b(i) is not only B'-X emission (primarily the dominant (0,O)diagonal band) originating from the initially populated B'ZA(v'=O) level, but also E X emission which arises from Sic1 molecules collisionally transferred to severalvibrational levels of the B22+state. Note that all vibronic bands are doubled due to theca. 200-cm-1 spin-orbit splitting in the X211 ground state. (Where necessary below we use the contracted notation XI^, X3/2 to label the respective spin-orbit states.) There are quite obviously striking differences between the spectra in Figure 2a,b. The ratio of emission from the collisionally populated B-state levels to that from the initially populated B'(v'=O) level is greater for H2 than for Ar, despite the similar pressures. The pattern of the E X emission bands (and hence the B-state vibrational populations) is also substantially different in the two cases. These two aspects were assessed more quantitatively by simulating the fluorescencespectra which would be observed for assumed relative populations of B'(u'-0) and B(v'=O,l,2). An initial estimate of the relative populations was made from the intensities of spectrally isolated EX(u',v'? bands and the E X (0,O)band. These were then refined by iterative adjustment until the agreement between synthetic and observed spectra was optimized at all wavelengths. The corresponding "best-fit" simulations of the results in Figure 2a(i),b(i) are given in Figure 2a(ii),b(ii), respectively. Further examples of the quality of the experimental data and the correspondingsimulationsfollowing initial excitationto B'2A(u'= 1) in the presence of Ar and Hzare presented in Figure 2c,d, respectively. Again there are obvious differences in the overall efficiency of transfer to the B state and in the resulting B-state vibrational populations. In this case the extent of direct B u ' = 3 state excitation on the EX312 (3,O) band which underlies the B'-X (1,O) QIhead (which is the strongest feature and was therefore used to obtain most of thedata, including those in Figure 2c,d) could be quantified directly. The B'-X3/2 (1,O) band, although weaker, is spectroscopicallyi~olated,~l allowing clean excitation of B' u' = 1 alone. Dispersed fluorescence spectra produced in this way containedessentially no measurableemission from B u' = 3, confirming that the very weak B u' = 3 features in spectra excited at the E X (1,O) Q1 head were entirely attributable to direct population, and not to collisional transfer. The simulation procedure was repeated for sequential experimental spectra recorded as a function of the pressure of each of the collidinggases for both u'= 0 and 1 of B'2A. The results were analyzed using a kinetic model in which Sic1 molecules initially prepared in B'zA(v'=O or 1) are assumed to have three possible fates. They can fluorescespontaneouslyto the X2ll ground state with unimolecular rate constant ~ / T B ( .Alternatively, they can be collisionally transferred to some vibrational level of the B22+
WAVELENGTH/nm
H2
E-X
E-X
LaasrJ U L 46
280 290 300 WAVELENGTH/nm
n
280 290 300 WAVELENCTH/nm
Figure 2. Dispersed fluorescence spectra following excitation to specific B'-state vibrational levels. The excitation wavelength, resonant with the QIhead of either the B'-X (0,O)or (1,O) band, as appropriate, is marked by thelabcl 'Laser": fluorescencesignals at this wavelength were partially obscured by scattered laser light and are not shown. Positions of the (doubled) B'-X and E X vibronic bands are indicated. In each case, i is experiment and ii is simulation using "best-fit" vibronic populations: (a)B'u'=O, l.STorrofAr;(b)B'u'=O, 1.5TorrofHzand0.75Torr of Ar; (c) B' u' = I, 1.5 Torr of Ar; (d) B' u' = 1, 1.5 Torr of H2 and 0.75 Torr of Ar.
III. Results Asdescribedin the Introduction,we haveprevi~usly~~ presented the spectroscopicinformation on the E X and B'-X transitions of Sic1 required in this study of collisional energy transfer. In summary,LIF excitation spectra allowed us to identify appropriate wavelengths at which we could selectively prepare B22+and B'2A vibrational (and to some extent rotational) levels. Dispersed fluorescence spectra revealed the characteristic pattern of vibronic band strengths from a given vibrational level of either the BZZ+ or B'ZA states. We were therefore in a position to deduce relative vibrational-state populationsfrom fluorescence spectra produced when more than one excited vibronic level was populated following a collisional transfer process. 111.1. Vibrationally Resolved Measurements. Our principal results are vibrationally-resolved fluorescence spectra observed following initial excitation to the u' = 0 and 1 levels of the B'2A state of Sic1 in the presence of various colliding gases. Those gases which we selected for ~ t u d y 3were ~ Ar, He, Hz, Nz, C02, CH,, and (partial results only) CFd. Parts a(i) and b(i) of Figure 2 show representative examples of spectra which result from excitation to B'2A(u'=O) in approximately equal pressures of Ar and H2, respectively. The excitation wavelength was resonant with the QIhead of the B'-X (0,O) band, with a pulse energy (-500 pJ cm-z) sufficient to significantly optically saturate this transition. This deliberate saturation had twin benefits. Power-broadening increased the range of rotational states populated in the B' u' = 0 state via
Singleton and McKendrick
1392 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993
state with a characteristic bimolecular rate constant, k ~ from , where they emit essentially instantaneously to the X2II ground state. Finally, they can be collisionally transferred to some state orher than B W with a rate constant, k ~from , where they do not emit at a wavelength observed in our experiments (Le. ca. 275325 nm). This kinetic scheme is easily solved to yield expressions for the time-dependent populations of the B’2A and B2Z+states. The rate of B’-X emission is at all times proportional to the product of the B’2A-state population and rB
