Resonance Raman spectra of CH2I2 have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2 in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2 exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I–C–I symmetric stretch, the I–C–I bend, and the I–C–I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time‐dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short‐time photodissociation...

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Solvation and solvent effects on the short‐time photodissociation dynamics of CH2I2 from resonance Raman spectroscopy

Resonance Raman spectra, including absolute scattering cross sections, have been measured for methyl iodide in hexane at nine excitation wavelengths from 204.2 to 274.0 nm. Spectra excited in the 246–274 nm region, on resonance with the directly dissociative A state, exhibit overtone progressions in the C–I stretch and its combination bands with the methyl umbrella mode. The fundamental of the C–I stretch is weak relative to the overtones when excited near the peak of the A band, an effect shown to arise from interferences between the resonant amplitude and preresonant contributions from higher electronic states. Both the absorption spectrum and the Raman excitation profiles are quite similar in solution and vapor phases, suggesting that the dissociative A state is not strongly perturbed by solvation. This conclusion is consistent with a simple theoretical estimate of the solvation effect on the A state potential.

Resonance raman excitation profiles of methyl iodide in hexane

Solvation effects and short-time photodissociation dynamics of CH2I2 in solution from resonance Raman spectroscopy

The photodissociation of (fluorinated) alkyl iodides in helium nanodroplets at a wavelength of 266nm has been investigated by means of ion imaging techniques. It is found that a significant fraction of the created fragments escapes from the helium droplets. The speed and kinetic energy distributions of these fragments are found to be notably modified with respect to the corresponding gas phase distributions. The fragments, furthermore, show a speed dependent angular distribution. The loss of kinetic energy as well as the reduction of the anisotropy parameter show a strong mass dependence. These observations point to a nonthermal escape process in which the kinetic energy and momentum transfer from the fragments to the solvent is governed by binary collisions with the individual helium atoms making up the droplet. Monte Carlo simulations based on hard-sphere binary collisions substantiate this interpretation of the data.

Photodissociation of alkyl iodides in helium nanodroplets. I. Kinetic energy transfer.

Photodissociation dynamics of ethyl iodide in the A band has been investigated at several wavelengths between 245 and 283 nm using resonance-enhanced multiphoton ionization technique combined with velocity map ion-imaging detection. The ion images of I, I-*, and C2H5 fragments are analyzed to yield corresponding speed and angular distributions. Two photodissociation channels are found: I(5p P-2(3/2))+C2H5 (hotter internal states) and I-*(5p P-2(1/2))+C2H5 (colder). In addition, a competitive ionization dissociation channel, C2H5I++h nu -> C2H5+I+, appears at the wavelengths < 266 nm. The I/I-* branching of the dissociation channels may be obtained directly from the C2H5+ images, yielding the quantum yield of I-* about 0.63-0.76, comparable to the case of CH3I. Anisotropy parameters (beta) determined for the I-* channel remain at 1.9 +/- 0.1 over the wavelength range studied, indicating that the I-* production should originate from the (3)Q(0) state. In contrast, the beta(I) values become smaller above 266 nm, comprising two components, direct excitation of (3)Q(1) and nonadiabatic transition between the (3)Q(0) and (1)Q(1) states. The curve crossing probabilities are determined to be 0.24-0.36, increasing with the wavelength. A heavier branched ethyl group does not significantly enhance the I(5p P-2(3/2)) production from the nonadiabatic contribution, as compared to the case of CH3I. (c) 2007 American Institute of Physics.

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Productions of I, I-*, and C2H5 in the A-band photodissociation of ethyl iodide in the wavelength range from 245 to 283 nm by using ion-imaging detection

Substituent effects on gas-phase photodissociation dynamics: resonance Raman spectra of ethyl iodide, isopropyl iodide, and tert-butyl iodide

Resonance Raman spectra of trans and gauche isomers of n-propyl iodide: Evidence for the dependence of photodissociation dynamics on geometrical conformation

Barbara A. Lawrence

Papers

Substituent effects on gas-phase photodissociation dynamics: resonance Raman spectra of ethyl iodide, isopropyl iodide, and tert-butyl iodide

Resonance Raman spectra of trans and gauche isomers of n-propyl iodide: Evidence for the dependence of photodissociation dynamics on geometrical conformation

Substituent Effects on Gas-Phase Photodissociation Dynamics: Resonance Raman Spectra of Ethyl Iodide, Isopropyl Iodide, and tert-Butyl Iodide