Evolution of molecular dissociation through an electronic curve crossing: Polarized emission spectroscopy of CH3I at 266 nm

Abstract

We present a study of the dissociation of CH3I on coupled repulsive electronic potential energy surfaces by the technique of polarized emission spectroscopy. We excite CH3I at 266 nm and disperse the photons emitted from the dissociating molecule by both frequency and angular distribution with respect to the polarization direction of the excitation laser. We thus measure the polarization of the first 12 C–I stretching emission features, corresponding to the spectral region between 266 and 317 nm. We also obtain the rotational envelope of selected emission features in higher resolution scans and model the lineshapes with parameters derived from the polarization results. The polarization measurements show the emission into the first few low-lying C–I stretching vibrational levels is via a transition moment parallel to the absorbing one, consistent with excitation to and emission from the 3Q0(2A1) repulsive surface. Emission to higher C–I stretching overtones shows an increasing contribution from emission via a transition moment perpendicular to the absorbing one, consistent with emission from a repulsive surface of E symmetry following excitation to the 3Q0(2A1) state. We extract from the data the fraction of photons emitted via a perpendicular transition for each of the C–I stretch emission features. The analysis includes the derivation of analytic expressions for the angular distribution of the photons, with and without integration over the rotational contour, when the detector has a finite acceptance angle. We discuss the results in relation to a simple model where photoabsorption excites the molecule to the 3Q0(2A1) repulsive surface (parallel transition moment) and amplitude develops on the 1Q1(3E) repulsive surface as the molecule dissociates through a curve crossing. The changes in amplitude of the molecular wavefunction on the A1 vs the E repulsive surfaces during dissociation is thus probed. We outline a crude classical quasidiatomic approximation for roughly extracting from our data the electronic energy at which the ‘‘curve crossing’’ occurs. This derived energy is compared to that given in model and ab initio calculations of the excited electronic potential energy surfaces. Finally, we discuss the results in relation to the simple quasidiatomic Landau–Zener crossing model utilized by other workers, a model which does not fully explain the collection of experimental results over the last decade on the iodoalkane curve crossing.

A -band methyl halide dissociation via electronic curve crossing as studied by electron energy loss spectroscopy

“…2͑c͒ at an incident energy of 30 eV and 20°scattering angle, where the 1 Q 1 perpendicular transition becomes more prominent. This is also reinforced by the polarized emission spectroscopy of Lao et al 29 at 4.66 eV ͑266 nm͒, who reported a gradual increase in emission from the 1 Q 1 potential energy surface on the n 3 overtones. This is also in agreement with the recent reported resonant feature of CH 3 I at 4.7 eV, in the vibrational excitation DCS of Kato et al 2 Nevertheless, Strobel et al 14 have indicated an increasing relevance of the coupling with a longer evolution time of the wave packet on the potential energy surface.…”

Dynamics of I*(2P1/2) production in the ultraviolet photodissociation of alkyl iodides

“…Hence the problem is simplified to the extent of finding the velocity at the crossing point. For this the translational energy (E,) at the asymptotic limits is calculated using a quasidiatomic model and is given by Can parameter, as pointed out by Lao et al (14). However, a detailed model to describe the dynamics will be complicated and c o nputationally demanding.…”

Short-time photodissociation dynamics of iodoalkanes: refinement using resonance Raman intensities of isotopic derivatives

“…This spec-CH 3 trum was qualitatively consistent with a picture of the initial photodissociation dynamics involving almost pure stretching of the CÈI bond and Ñattening of the dihedral angle at later times. The polarization CH 3 properties of the resonance Raman scattering of iodomethane have also been examined by both Lao et al 50 and more recently Wang and Ziegler51 in order to quantitate the contributions from electronic states other than Resonance Raman spectra of iodomethane 3Q 0 . have been obtained in several solvents to examine environmental e †ects on the early time photodissociation dynamics by Myers and Markel.52h54 Resonance Raman spectra have also been obtained for both vapour-and solution-phase iodoethane, 2-iodopropane and tert-butyl iodide.55h57 The solutionphase study56 reported both relative and absolute resonance Raman cross-sections over a wide range of excitation wavelengths and showed that the vibrational fundamentals in the A-band resonant Raman spectra derive much of their intensity from higher electronic states.…”