J. CHEM. SOC. FARADAY TRANS., 1994, 90(11), 1467-1471 1467 Collisional Behaviour with Ar of the A Doublets of CH(X 'n)N" = 15 produced in the Two-photon Dissociation of CH2C0 at 279.3 nm Stephen M. Ball, Graham Hancock* and Mathew R. Heal? Oxford Centre for Applied Kinetics, Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford,UK OX1 3QZ Relaxation of the N" = 15 A doublets of CH(X211) produced in the two-photon dissociation of ketene at 279.3 nm has been observed in the presence of Ar. An initially equilibrated nascent A doublet population is seen to be removed in such a way that the component of A" symmetry (n orbital perpendicular to the plane of rotation) dominates. The application of recent theory describing collisions of 'll diatomics with closed shell atoms is brief I y discussed.The CH radical is one of the most ubiquitous and reactive of all free radicals and plays a key role in the chemistry of com-bustion and atmospheric systems. In addition to its kinetic importance in elementary reaction steps, CH is of fundamen- tal interest to theoreticians and experimentalists studying the dynamics of collisional quenching and energy transfer pro- cesses of small radical species. In particular, state-resolved observations of the population of the A doublets of the radical yields valuable information on the stereochemical behaviour of the singly occupied 71 electron orbital with respect to the rotating molecular framework. An initial bias towards population in specific A doublets has been noted in the dispersed emission of nascent rota- tional levels in the first excited electronic state of CH('A), a commonly observed product from the multiphoton disso- ciation of CH containing polyatomics. For example, Stuhl and co-workers' have observed a propensity for population of CH('A) in rotational levels of II(A') symmetry (pn orbital parallel to the diatomic rotating plane) following the 193 nm photolysis of acetone.This is in partial agreement with Nagata et aL2 for the same system who report a switching to ll(A") propensity for rotational levels greater than N' x 20. Similar experiments with ketene, CH,CO, as a precursor at a photolysis wavelength of 193 nm, also report a higher inten- sity emission in the symmetric A component for rotational levels with N' = 14-19, but the opposite for levels N' = 20-The present work focuses on the A doublet populations of the ground electronic state.We have recently reported mea- surements on the nascent rotational populations for ground- state CH(X 'll) produced from the two-photon photolysis of ketene at wavelengths of 279.3 and 308 nm.4 At 279.3 nm the nascent rotational distribution probed by LIF exhibits equal population in the A doublets, while at 308 nm there was a slight degree of orbital alignment in favour of II(A') sym- metry for the higher rotational levels populated. The likely dissociation pathways leading to these results were discussed. Here we report on the subsequent evolution of the CH(X 211) rotational populations in collisions with Ar.We observe a significant inequality in the initial behaviour of the popu- lations of the two A symmetries with respect to both time and pressure of Ar. These observations must indicate a pref- erential rotational energy transfer mechanism, dependent on 71 orbital symmetry, that operates for certain rotational levels of CH in collisions with the inert gas. The results are dis- cussed in terms of the extensive theoretical treatment for inelastic scattering of 'II diatomics which has been developed by Alexander and co-~orkers.~-' t Present address: School of Chemistry, University of Leeds, Leeds, UK LS2 9JT. Experimental In these experiments a standard LIF detection apparatus was used which has been described in more detail The CH(X 'II) radical was generated by the two-photon pho- tolysis of ketene, CH,CO, at 279.3 nm,4 and detected by on- resonance LIF within the Q and R branches of the CH A'A-X'll system at wavelengths between ca.418 nm and 432 nm." Plane polarised photolysis light at 279.3 nm was produced using the frequency-doubled output of a Quanta Ray 5200 dye laser pumped by a XeCl Questek 2240 excimer. The probe laser radiation was obtained either from a Molec- tron Corp. UV 24 nitrogen laser pumped Molectron DL 200 dye laser (energies up to ca. 50 pJ pulse- ',bandwidth ca. 1.0 cm-') or a Lambda Physik excimer laser pumped dye laser, EMG101/FL2002 combination (output up to 10 mJ pulse- and bandwidth of ca.0.4 cm-I). The LIF was observed per- pendicular to the orthogonal intersection of the horizontal photolysis and probe laser beams using an EM1 9813QKB photomultiplier tube and an appropriate interference filter. Data were acquired either by gating and integrating the signal using a Brookdeal 9415/9425 boxcar combination and a chart recorder or by digitisation via a 20 MHz Thurlby DSA524 Digital Storage Adaptor and a PC. Ketene precursor was prepared as described previously by the pyrolysis of acetone vapour in He passed over an electri- cally heated nichrome element at ca. 650°C.'' Purity was always checked by mass spectrometry and UV absorption between 200 and 400 nm. When not frozen the ketene was stored at pressures less than 20 Torr in a darkened bulb to prevent polymerisation. Argon (Ar) diluent was obtained from BOC with a stated purity of 99.995% and used as received.Experiments were conducted using a static gas sample within the stainless steel cell of ca. 30 mTorr CH,CO and variable partial pressures of Ar bath gas. Each rotational level within the CH(X 211) and CH(A 2A) manifolds is split by spin-orbit coupling [Hund's case (b)] and by A doubling, although the magnitude of the splitting is considerably greater in both instances for the ground state. The selection rules allow transitions within the A-X system from all four levels associated with a given lower state rota- tional quantum number N".' ,The fine-structure populations can therefore be obtained directly from a LIF scan within a single rotational branch.For CH(X 21'1) with its relatively large rotational constant (Be = 14.46 cm-' 13) but weak spin-orbit coupling constant (A = 28.14 cm-' lo), the A splitting rapidly dominates the spin-orbit splitting as rota- tional energy increases. The bandwidth of the Molectron laser as probe was sufficient to resolve the A doublets but not the spin-orbit components contained within each. In order to standardise the notation associated with the rotational levels J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 R(0,O)N" I I I I I 418 420 42 2 424 426 excitation wavelength/nm Fig. 1 (a)LIF spectrum in the R branch region of the A 2A-X211transition for nascent CH(X 2n)produced in the two-photon dissociation of ketene at 279.3 nm using the Molectron nitrogen laser pumped dye laser combination as probe.Positions of the R branch transitions in the (0,O) band are indicated as a function of N". For each N" doublet the transition at lower wavelength probes the levels of n(A)symmetry, whilst that at higher wavelength probes the levels of n(A)symmetry. Partial pressure of ketene 30 mTorr. (b) As for Fig. l(a), but under collisional conditions of 5 Torr pressure of Ar and a photolysis-probe delay of 0.3 ps (equivalent to ca. 15 gas kinetic collisions). The preferential population of n(A") levels under collisional conditions is clearly seen for N levels 3 12. of such systems Brown et al.12 and Alexander et all4 have These correspond to the singly occupied x orbital of the classified the fine-structure levels according to the behaviour radical being aligned parallel or perpendicular to the plane of of the electronic wavefunction on reflection in the plane of rotation, respectively.For the N" = 15 levels of CH(X211), rotation of the diatomic in the limit of high J. As the case (b) the subject of the present study, the R branch transition con- limit is approached the F,, and F,, wavefunctions of a ,ll sists of a doublet, the higher wavelength component of which state acquire symmetric character with respect to this reflec- corresponds to transitions from the unresolved F,, and F,, tion and are denoted ll(A') while the F,, and F,, wavefunc-levels, each of ll(A') symmetry, with the lower wavelength tions acquire antisymmetric character and are denoted ll(A"). component being from F,, and F,, levels, both of ll(A'l) sym- J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 01 I I I I I I ' I I ' I 01 2345678910 probe delay/ps Fig. 2 Semi-logarithmic plot of the population af both components of the N" = 15 A doublet of CH(X 'n) as a function of increasing time delay following photolysis at 279.3 nm. Partial pressures of CH,CO and Ar were 30 mTorr and 1.0 Torr, respectively. The squares correspond to CH rotational levels of n(A") symmetry and the circles to levels of n(A') symmetry. metry. This part of the spectrum thus gives a convenient probe of the A doublet behaviour. Two distinct pumpprobe beam relative polarisation geometries could be investigated, either both electric vectors parallel to each other and vertically orientated with respect to the laboratory frame or alternatively with the polarisation of the photolysis beam rotated through 90". At short pump- probe delays copious amounts of nascent 'background' emis- sion from excited CH(A2A) had to be subtracted from the LIF signal of interest and significantly impaired sensitivity.Results Previous work in this laboratory has shown that the nascent population of CH prepared by the two-photon photolysis of ketene at 279.3 nm clearly has a 1 : 1 ratio in A doublet population for all N" levels probed in the R branch region between 418 and 426 nm.4 The LIF spectrum is reproduced here in Fig. l(a) for comparison. The corresponding spectrum of CH under collisional conditions (5 Torr Ar at a delay of 0.3 ps, corresponding to ca.15 gas kinetic collisions) is illus- trated in Fig. l(b). Here there are obvious unequal A doublet intensities for all rotational levels N" 2 12. 0.51 I I I0 5 10 15 probe delay/ps Fig. 3 The variation in ratio of population of n(A)and n(A') com- ponents of the N" = 15 A doublet of CH (photolysis at 279.3 nm) as a function of probe delay at a fixed Ar pressure of 0.5 Torr. The circles correspond to both photolysis and probe laser beams verti- cally plane polarised with respect to the laboratory frame, and the squares to a 90" relative polarisation geometry. The n(A):n(A')ratio rises to a limiting value in a way that is independent of polari- sation geometry.The increasing error bars with time reflect the reduction in the magnitudes of the absolute LIF signals as CH is removed. For this effect to be attributed to unequal A doublet popu- lations in the ground 211 state, the influence of different exci- tation and fluorescence rates needs to be considered. First, lifetimes T of the 'A N' = 16 levels accessed in the transition were found to be represented by 7 = 560 40ns for Ar pres- sures between 0 and 5 Torr, in agreement with previous mea~urements'~-'*and eliminating the effect of quenching of the upper 'A level on the fluorescence intensities. Secondly, the A doublet ratio was found to be independent of relative polarisation of the pump and probe beams, showing that alignment effects were negligible.Finally, the ratio remained constant over a ten-fold change in probe laser intensity, eliminating saturation effects in the transition. The propen- sity is thus a phenomenon occurring within the ground state of CH, with the propensity for the shorter wavelength com- ponent of the transition for a given N" indicating a greater population for CH of lI(A") symmetry. From Fig. 1 it can be seen that although disequilibrium between the A doublet levels has been established in a time corresponding to ca. 15 gas kinetic collisions, the overall rotational state population is not markedly affected, i.e. the two rotational distributions for Fig. l(a) and (b) peak at approximately the same value of N".It should also be noted that the disequilibrium effect is most apparent at high N". Rotational levels with N" < 12 appear to be unaffected under the collisional conditions. Fig. 2 shows the time dependence of the two A doublets in the presence of Ar. Both components decrease monotonically with time, but it can be seen that a disequilibrium between the two is established rapidly (< 1 ps for a pressure of 1 Torr Ar as shown in Fig. 2) and is maintained over a timescale of several lower state lifetimes. Fig. 3 shows the evolution of the A doublet ratio as a function of time at a pressure of 0.5 Torr Ar and 30 mTorr ketene, reaching a limiting value of ca. 1.7 in a timescale corresponding to ca. 20 Ar collisions. Under these conditions, CH(X 'n) is removed predominantly by reaction with ketene,9 but the effect was shown to be depen- dent on the presence of Ar by experiments in which the A doublet ratio was found to be invariant (1.1 & 0.1) over the same timescale with ketene alone, and to show a similar increase to a limiting value when measured as a function of Ar pressure at constant time delay and ketene pressure.Our conclusions about the anomalous behaviour are as follows: (1) The relaxation of a 'hot' distribution of CH rota- tional states initially in A doublet equilibrium in collisions with Ar (when chemical removal by reaction with ketene also occurs) takes place through a marked A doublet disequi- librium. (2) The effect is not solely due to preferential chemi- cal removal of a single A doublet; Ar collisions are necessary.(3) The persistence of the disequilibrium after the photolysis pulse is over shows that the effect is not dominated by col- lisional processes affecting the initial A doublet population. In previous studies it was proposed that the ketene disso- ciation process involved single-photon absorption to a pre- dissociative state followed by a second absorption step.4 Any collisional relaxation of the intermediate to a lower-energy longer-lived state might affect the subsequent energy distribu- tion (and possibly the A doublet ratio). A collisional effect on the nascent CH quantum yield was seen, but as Fig. 2 and 3 show, the disequilibrium effect persists at timescales orders of magnitude greater than the duration of the photolysis pulse.Discussion We first note that the ketene system is not the first in which such anomalous A doublet ratios have been seen. In an experiment very similar to this, Stuhl and Heinrich' have observed an almost identical effect on the N” = 15 A doub-lets of CH found in the multiphoton dissociation of CH, Br, at 193 nm. An initially equal ratio of A doublets increased to a n(A”): II(A’) ratio value of cu. 2.5 with Ar collisions, i.e. with both time and pressure of inert diluent. The collisional behaviour described in the present work is a quenching for CH incorporating a preference for A doublets of II(A”) sym-metry, regardless of the nascent population of A doublets that has been produced on photolysis.This bias is for the electron density of the d electron to lie parallel to the total angular momentum vector J, and perpendicular to the plane of rotation of the radical, in the limit of high angular momen- tum.I4 Macdonald and Liulg have used a cross molecular beam apparatus to collide CH N = 1 radicals with He atoms at collision energies up to 12 kJ mol-’. An initially equal dis- tribution of all four fine-structure states of N = 1 resulted in preferential rotational energy transfer of population to the If and 2e states [of ll(A”) symmetry] in levels of N > 1, as com-pared to population in the le and 2f states of the same final levels, Towards the upper end of collision energies, the ratio of the population of the two symmetries of CH in the excited rotational levels was typically n(A”) : II(A’) = 2.5 : 1.The ratio increased exponentially with decreasing energy as the threshold for the excitation process was approached. The probe-laser resolution available to us in the present work was insufficient for such full characterisation of all fine-structure states. We are also concerned wth rotational quenching of the CH radical in downward transitions, rather than the mea- surement of differential cross-sections for net inelastic energy transfer to the radical in upward rotational transitions. Theoretical work on a general formalism for inelastic scat- tering of a diatomic radical with a structureless collision partner has been extensively developed by Alexander and co- worker~.~*~**~~~In these open-shell systems the nearly degen- erate potential-energy surfaces at long range may interfere as a collision partner approaches, and prevent treatment of the collision as an event taking place on a single surface.Com- plications arise also from different coupling systems of the various momenta. Calculations have been applied to the spe- cific example of inelastic scattering and resultant orbital alignment of CH in collisions with He,8 and general agree- ment found with the molecular beam result^.'^ The non- statistical population of the final state A doublet levels arises directly from an interference between the scattering ampli- tudes on the two potential-energy surfaces upon which inelas- tic collisions occur.Approach of a spherical scattering partner to the ’ll radical lifts the electronic degeneracy of the 211state giving a total wavefunction for the system that is either symmetric A’ or antisymmetric A” with respect to reflection of the electronic spatial coordinates in the triatomic plane. Collisions are described using average and difference linear combinations of these surfaces, i(VA.+ VAj,)and i(VArr -VA!),respectively. In a Hund’s case (b) diatomic, where neither L nor S is strongly coupled to the internuclear axis, both the average and difference potentials can contribute to both fine-structure conserving collisions (influence of the average potential) and fine-structure changing collisions (influence of the difference potential).Interference between the scattering amplitudes of the different paths which lead to the same final spin-orbit state imposes a bias in differential cross-section and a propensity in A doublet population. The preferred final symmetry is dependent on the relative signs and magnitudes of the average and difference potentials. A single theoretical calculation for loss of rotational energy in the CH + He system, J = 3,F, +J = 4, F,, predicts a dif- ferential cross-section ratio of cA,,/cA~ = 0.53,’ i.e. preferential de-excitation into states of II(A’) symmetry. This prediction J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 has not been tested experimentally and is opposite to our findings presented here.The reason for the discrepancy pre- sumably arises from the different functional form of the potential-energy surfaces required to describe collisions of CH with a much higher degree of rotational excitation and with a heavier Ar scattering partner as investigated in the present work. A change in relative signs of the average and difference potentials will affect the symmetry of the states [to II(A) for de-excitation collisions] of a particular level into which CH is preferentially quenched. The long-range disper- sion interaction in the collision is proportional to the polar- isability of the spherical scattering partner and will be strongest when the 71 electron of CH lies in the orbital within the triatomic plane defined by the collision.Since the polar- isability of an Ar atom is almost an order of magnitude greater than that of He, the signs of the dispersion interaction for the two symmetries may play an influential role in the overall balance of scattering amplitudes. A further consideration is that the entire scattering process may be a J (or N) dependent quantity. This would account for the observed discrepancy that A doublets with N” > 12 in the LIF spectrum under collisional conditions for CH pro- duced at 279.3 nm photolysis show a collisional propensity while levels of lower rotation continue to exhibit the nascent equilibrium population [Fig. l(b)]. Alternatively, this obser- vation may be a manifestation of the system striving towards the equilibrium that would be eventually attained in all ther- mally populated levels at ambient conditions were it not for the population of CH being removed by chemical reaction before an equilibrium can be established.Evidently, there is a requirement for further detailed studies on the behaviour of many more A doublets and spe- cific quenching rates through individual rotational levels. In particular, fully time-resolved data for decay rates of fine structure components at fixed pressures of Ar and a thorough investigation at additional photolysis wavelengths are required to unravel completely the mechanisms underlying the dissociation pathways to, and collisional quenching of, the resulting CH fragment. The award of a studentship to M.R.H. by the SERC is grate- fully acknowledged. References 1 P.Heinrich and F. Stuhl, 1990, unpublished results. 2 T. Nagata, M. Suzuki, K. Suzuki, T. 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