Highlights

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The first study for time scales of photodissociation dynamics in iodocyclohexane.

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The dissociation dynamics within A-band is prompt and direct.

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The dissociation time within A-band is around 180 fs.

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The C-state has predissociative character with reaction time of about 600 fs.

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All time constants elongate as the 800 nm probe pulse intensity enhances.

Highlights

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The internal conversion from S₂ state to hot S₁ state is observed.

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The lifetime of initially prepared S₂ state was determined to be about 72 fs.

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The secondary populated S₁ state decays within timescales 16.4 ps.

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The lifetime of S₂ was slightly increased by methylation comparing with benzene.

Abstract

The photodissociation dynamics of cyclopentyl bromide at 234 and 266 nm were investigated using velocity ion imaging. Translational energy distributions of Br and Br^∗ have been fitted by two Gaussian functions. It is possible that they are originated from different conformational structures. Three Gaussian functions are required to fit the distributions of Br^∗ at 266 nm, which is attributed to the multiphoton dissociative ionization. The rigid radical limits of the impulsive model have been applied to the related energy partitioning. The branching ratios and the relative quantum yields were determined; the results indicated that ground-state bromine was the major dissociation product.

Abstract

The dissociative ionization of ICl after irradiation with a femtosecond laser pulse centered at 800 nm has been investigated. The speed distributions of produced I⁺ ions at a series of laser intensities have been measured and analyzed by velocity map imaging method. All of them can be fitted using four Gaussian functions. Four production channels are observed for I⁺ fragments, which originate from dissociation of A  ²Π_i${}^2{\mathrm{\Pi }}_i$ or B  ²Σ⁺${}^2{\mathrm{\Sigma }}^{+}$ states when the molecule is excited above the ionization threshold by multi-photon absorption. The images have shown obvious anisotropic character along the laser polarization.

The photodissociation of ethyl bromide has been studied in the wavelength range of 231–267 nm by means of the ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton ionization (REMPI) scheme. The velocity distributions for the Br (²P_1/2) (denoted Br*) and Br (²P_3/2) (denoted Br) fragments are determined, and each can be well-fitted by a narrow single-peaked Gaussian curve, which suggests that the bromine fragments are generated as a result of direct dissociation via repulsive potential-energy surfaces (PES). The recoil anisotropy results show that β(Br) and β(Br*) decrease with the wavelength, and the angular distributions of Br* suggest a typical parallel transition. The product relative quantum yields at two different wavelengths are Φ_234nm(Br*)=0.17 and Φ_267nm(Br*)=0.31. The relative fractions of each potential surface for the bromine fragments’ production at 234 and 267 nm reveal the existence of a curve crossing between the³Q₀ and¹Q₁ potential surfaces, and the probability of curve crossing decreases with the laser wavelength. The symmetry reduction of C₂H₅Br from C_3vto C_sinvokes a nonadiabatic coupling between the³Q₀ and¹Q₁ states, and with higher energy photons, the probability that crossing will take place increases.

Abstract

The photodissociation of C₂H₅Br and n-C₃H₇Br at 234 and 267 nm was investigated utilizing time-of-flight (TOF) mass spectrometry coupled with resonance-enhanced multiphoton ionization (REMPI) scheme. The experiment found that the branching ratio Br*(²P_1/2)/Br(²P_3/2) of C₂H₅Br decreased, and that of n-C₃H₇Br increased with the laser changing from 267 to 234 nm. In order to interpret the experimental results, ab initio calculations were performed to get potential energy surface (PES) curves of the compounds. The calculations predicted an avoided crossing between the PES curves of C₂H₅Br, which would result in non-adiabatic dissociation of C₂H₅Br, while no curve crossing among the PES curves of n-C₃H₇Br, leading to adiabatic dissociation of n-C₃H₇Br. The calculated results matched the experimental findings very well.

Abstract

Mass-analyzed threshold ionization (MATI) spectroscopy and two-color resonant two-photon ionization method were used for the determination of the vibrational levels of the p-xylene cation. The MATI spectrum was recorded via the 0⁰ vibrationless level of the S₁ state of p-xylene. The spectrum shows a rich structure and some vibrational frequencies of the cation are determined. The experimental findings are well supported by ab initio calculation.