Photodissociation of CD₃SCD₃ on the First Absorption Band:  Translational and Internal Energy Transfer to the CD₃ Fragment Studied by Resonant Multiphoton Ionization and Time-of-Flight Spectrometry

Abstract

The photodissociation of deuterated dimethyl sulfide (CD 3 SCD 3 ) has been studied in the first absorption band at a laser wavelength of 229 nm. The resonance-enhanced multiphoton ionization time-of-flight technique has been employed to determine the recoil energy distribution and the anisotropy parameter and to study the rovibrational state population of the nascent CD 3 . The observed value ) -0.9 ( 0.1 (perpendicular dipole transition) corroborates the C 2V 1 B 1 (or C s 1 A′′) symmetry of the excited state accessed at 229 nm, as assigned previously by different groups. For dissociation yielding vibrationless CD 3 , the center-of-mass translational energy is, on average, 80% of the maximum energy available. CD 3 (V)0) fragments are produced with a rotational distribution showing a maximum at N′′ ) 9-11. Evidence for activity of the ν 1 symmetric stretching mode of CD 3 is also observed. These results are compared with those obtained for deuterated methyl iodide (ICD 3 ). I. IntroductionDimethyl sulfide (DMS) has attracted the attention of scientists during the past decades, largely due to its implication in the tropospheric sulfur cycle. 1-3 In particular, the enhancement of DMS oxidation upon irradiation with ultraviolet (UV) light 3 has motivated a series of detailed photodissociation studies. 4-8 Several experiments of this kind were performed in the early 1990s by Lee et al. 7 using molecular beam translational spectroscopy and by Nourbakhsh et al. 6 employing, additionally, photoion-photoelectron coincidence spectroscopy. These experiments, which focused on the 193 nm photodissociation of CH 3 SCH 3 , confirmed C-S bond breaking as being the dominant channel taking place upon UV absorption, and showed that a substantial part of the energy available (about 60%) is transferred into translational energy of the CH 3 and CH 3 S photoproducts. Furthermore, both fragments were found to be scattered without any preferential recoil direction and this was interpreted as being a consequence of the many excited states involved in the photodissociation of this molecule at 193 nm.We have recently undertaken the study of the photodissociation of DMS following laser excitation at longer wavelengths (227-229 nm). 8 The absorption at these wavelengths is dominated by the first dipole allowed transition in C 2V symmetry, 1 1 B 1 (9a 1 r 3b 1 ) r X 1 A 1 . 1,9-12 Therefore, one electron in the highest occupied molecular orbital (HOMO) in the ground-state configuration, the 3b 1 nonbonding orbital, which is essentially the 3p x atomic orbital of sulfur perpendicular to the C-S-C plane, 9 is promoted to the weakly bonding 9a 1 sulfur 4s Rydberg orbital.

Photodissociation Dynamics of Dimethyl Sulfide Following Excitation within the First Absorption Band

“…Alternatively, the forward convolution method of Bergmann and coworkers 26 can be used. Application of eq 6 to the magic-angle TOF profile, I(V Z′′ ,54.7°), provides an estimate of the speed distribution g(V), which is then used to determine the form of a representative velocity distribution obtained via the transformation g(V) ) P(E rec )|dE rec /dV| from a modified Gaussian recoil energy distribution function 6 where x ) E rec /E max is the fraction of the total available energy, E max , appearing as fragment translational recoil energy; x 0 and ∆x are fit parameters; and N is a normalization constant. Convolution of P(x) with the instrument response function yields simulated TOF profiles that can be compared with experimental results.…”

“…Because more than one excited state must usually be considered upon absorption at ultraviolet wavelengths, the influence of nonadiabatic interactions on product observables has attracted particular attention. An interesting example that has been the subject of recent theoretical 4 and experimental [5][6][7][8][9] work is provided by the photodissociation of CH 3 SCH 3 (DMS) in its first absorption band (∼215-230 nm). The electronic configuration of DMS in its ground state can be written as where the highest occupied (nonbonding) 3b 1 molecular orbital is dominated by the 3p x orbital of atomic sulfur, directed perpendicular to the C-S-C plane.…”

“…More recently, a number of experimental studies have focused on the dissociation following excitation to the first absorption band using the resonance-enhanced multiphoton ionization timeof-flight (REMPI-TOF) spectroscopy 5,6 and velocity map ion imaging 7,8 techniques. Spectroscopy of the nascent methyl fragments, with the analysis of time-of-flight (TOF) profiles and ion images, indicated that approximately 80% of the total available energy is deposited into fragment translation, approximately 5% is disposed into internal CH 3 modes, and the remaining 15% appears as internal excitation of the SCH 3 fragment.…”

“…These findings allowed the salient features of the dissociation process to be surmised, suggesting a prompt dissociation mechanism that induces effective energy transfer into fragment translation. [5][6][7][8] Although the energy partitioning and anisotropy provide a coarse-grained picture, understanding of the photodissociation mechanism can be greatly enhanced by specifying the correlation between the vectors J, the angular momentum, and v, the recoil velocity of the fragments, in a molecular frame defined by µ, the transition dipole moment of the parent molecule at the time of excitation. To this end, a recent study 9 of DMS in the first absorption band (at 219 nm) sought to characterize the laboratory-frame (v, J) alignment of the methyl photofragments immediately after dissociation.…”

Resonance-enhanced multiphoton ionization spectroscopy of CH3 and CD3. Two-photon absorption selection rules and rotational line strengths of the ν3- and ν4-active vibronic transitions

“…Resonance-enhanced multiphoton ionization (RE-MPI) spectroscopy of the methyl radical [1][2][3][4] has been proven to be a powerful technique to study the photodissociation dynamics of its precursors [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. To extract information on the population distribution of methyl radicals in various rotational states, the rotationally resolved 0 0 0 vibronic band of the two-photon transition jnp 2 A 00 2 i jX 2 A 00 2 i (n ¼ 3 or 4) in the REMPI spectrum has been thoroughly investigated [2][3][4]8].…”