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

Abstract

The technique of resonance-enhanced multiphoton ionization with time-of-flight mass spectrometry (REMPI-TOF MS) has been used to study the photodissociation of fully deuterated dimethyl sulfide (CD 3 SCD 3 ) following excitation at several wavelengths within the first absorption band (215-229 nm). Analysis of measured time-of-flight profiles of the nascent CD 3 products indicates a strongly anisotropic photodissociation, with approximately 70-80% of the available energy appearing as fragment recoil translation. A hybrid statistical-impulsive dissociation model predicts a photolysis wavelength dependence of the translational energy release that compares favorably with experiment, supporting the suggestion (Manaa, M. R.; Yarkony, D. R. J. Am. Chem. Soc. 1994, 116, 11444) that both the 1 1 A′′ and 2 1 A′′ excited states play an important role in the photodissociation process. An analysis of nascent CD 3 (µ, v, J) vector correlations, where the fragments are assumed to recoil axially along the CD 3 S-CD 3 bond, adequately accounts for the observed probe laser polarization dependence of velocity-selected REMPI spectra. The weak observed rotational excitation of the methyl fragments originates from zero-point vibrational motion of the dissociating parent molecule.

Photodissociation Dynamics and Stereodynamics of Methyl Mercaptan and Dimethyl Sulfide from the Second Absorption Band at 201 and 210 nm

“…A lifetime of ∼100 fs for the DMS excited state was obtained through the fit of the individual vibronic bands to Lorentzian profiles. In extraordinary agreement with the work of Aoiz and co-workers, 8 the translational partitioning ratio was found to decrease with the excitation energy. 11 The authors suggest, however, that the short lifetime of ∼100 fs makes unrealistic a statistical contribution to the dissociation process.…”

“…A similar estimation made for the process at 201 nm provides values of 2.2 eV for E av and ∼1.63 eV for E T , resulting in an average of ⟨ E T / E av ⟩ ∼ 0.74. This result, although in agreement with the values obtained in the literature for the photodissociation of DMS in a wide interval of photolysis wavelengths, ,, apparently do not match with the trend found. In an earlier set of experiments carried out by Aoiz and co-workers between 215 and 231 nm, − the fraction of the available energy transferred into translational motion was found to decrease with the excitation energy.…”

“…Furthermore, the recoil energy distributions showed that the available energy channeled into translation of the fragments decreases as the excitation energy of the parent molecule increases, which is an indication that the internal energy of the methylthio fragment increases as the photolysis wavelength decreases. Those results were rationalized in terms of a hybrid statistical-impulsive dissociation model …”

“…Those results were rationalized in terms of a hybrid statistical-impulsive dissociation model. 8 The analysis of the dissociation anisotropy of DMS provided β parameters close to the limiting value of −1 (for a pure perpendicular transition) while the full μ−v−J vector correlation analysis, i.e., the correlation between the transition dipole moment μ, the fragment recoil direction v, and the total angular momentum J of the detected fragment, indicated that the J vector does not have cylindrical symmetry about the recoil velocity v. 8,9 Such result is in disagreement with a pure impulsive model, which considers only in-plane and out-ofplane rocking modes. It was suggested that the parent ν 3 C− S−C deformation mode, tentatively introduced in the hybrid model, could explain the alignment of the J vector.…”

Use of the spatial phase of a focused laser beam to yield mechanistic information about photo-induced chemical reactions

“…The photodissociation dynamics of DMS has been studied extensively in the near UV [21], ( [22] and references cited therein). The primary excitation step for wavelengths >190 nm is the promotion of a non-bonding sulfur electron (from 3b 1 , the highest occupied molecular orbital) to a 4s Rydberg-like orbital (9a 1 ) or a valence-like C-S anti-bonding (6b 1 ) orbital [23,24] 3 .…”