Time-resolved predissociation of the vibrationless level of the B state of CH3I

The predissociation dynamics of the vibrationless level of the first Rydberg state 6s (B 2E) state of CH3I has been studied by femtosecond-resolved velocity map imaging of both the CH3 and I photofragments. The kinetic energy distributions of the two fragments have been recorded as a function of the pump-probe delay, and as a function of excitation within the umbrella and stretching vibrational modes of the CH3 fragment. These observations are made by using (2+1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the 3pz 2A2" state of CH3 to detect specific vibrational levels of CH3. The vibrational branching fractions of the CH3 are recovered by using the individual vibrationally state-selected CH3 distributions to fit the kinetic energy distribution obtained by using nonresonant multiphoton ionization of either the I or CH3 fragment. The angular distributions and rise times of the two fragments differ significantly. These observations can be rationalized through a consideration of the alignment of the CH3 fragment and the effect of this alignment on its detection efficiency. Two extra dissociation channels are detected: one associated with Rydberg states near 9.2 eV that were observed previously in photoelectron studies, and one associated with photodissociation of the parent cation around 15 eV.Comment: submitted Physical Chemistry Chemical Physics (2011

Laser-induced saturated fluorescence as a method for the determination of radical concentrations in flames

Kohse-Höinghaus K, Perc W, Just T. Laser-induced saturated fluorescence as a method for the determination of radical concentrations in flames. Berichte der Bunsen-Gesellschaft. 1983;87:1052-1057

Ultraviolet photodissociation dynamics of H2S and D2S

Nascent SH(X2 Πi, v = 0,1) and SD(X2 Πi, v = 0,1) rotational state population distributions, spin-orbit state population ratios, and A-doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ = 193, 222, and 248 nm) and D2S (λ = 193 and 222 nm), respectively. Nascent SH(X2 Πi, v = 0) rotational state distributions following 193 nm photodissociation of cold H2 S in a free jet expansion vs 300 K H2 S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X2 Πi, v = 1) and SD (X2 Πi, v = 1) have been recorded for the first time. Rotational state distributions for SH(X2 Πi, v ) and SD(X2 Πi, v ) are independent of v . A-doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consistent with an impact parameter model for the dissociating triatomic molecule

LASER INDUCED FLUORESCENCE OF HCN FROM THE A~1A′′\tilde{A} {^{1}}A'' STATE

Author Institution:Fluorescence spectra of the near vaccum ultraviolet $\tilde{A} ^{1} A^{\prime\prime} \rightarrow \tilde{X} ^{1}\Sigma^{+}$ transition of HCN have been recorded and assigned. A tuned (0.15A bandwidth) ArF excimer lasser is used to excite the (010)$^{1}$, (000)$^{2}$ and (000)$^{0}$ individual vibronic features in the $\tilde{A} ^{1} A^{\prime\prime}$ state. Fluorescence terminating on high vibrational levels of the $\tilde{X}^{1}\Sigma^{+}$ state dominates the emission spectra. Progressions in the $v_{2}$ bending and $v_{3}$ stretching modes are assigned. The fluorescence spectra, emission assignments, fluorescence lifetimes and the fluorescence excitation spectrum will be reported and discussed

LIFETIMES OF THE \~{B} STATE OF CH3ICH_{3} I AND CD3ICD_{3} I USING FEMTOSECOND RESONANCE ENHANCED MULTIPHOTON IONIZATION

Author Institution: Code 6111, Chemistry Division, Naval Research LaboratoryFemtosecond pulses in the deep uv (near 200 nm) have been used to directly measure The $\widetilde{B}$ state lifetimes for gas phase $CH_{3}I$ and $CD_{3}I$ by time-resolved MPI. Tunable deep uv femtosecond excitation pulses were generated by sum frequency generation in BBO of a frequency doubled CPM laser with amplified portions of continuum. For the vibrationless state of $CH_{3}I (CD_{3}I)$ the lifetime is 1.38 ps (1.84 psec). It is longer for the $\nu_{3}=1$ level of $CH_{3}I$ (1.80 ps). These values indicate the multidimensional nature of the dissociation and their isotopic and mode dependence will be compared with those obtained by indirect measurements