Multiphoton Ionization/Dissociation of Molecular Sulfur S2 in the UV Region.

The multiphoton ionization/dissociation dynamics of molecular sulfur (S2) in the ultraviolet range of 205-300 nm is studied using velocity map ion imaging (VMI). In this one-color experiment, molecular sulfur (S2) is generated in a pulsed discharge and then photodissociated by UV radiation. At the three-photon level, superexcited states are accessed via two different resonant states: the B3Σu- (v' = 8-11) valence states at the one-photon level and a Rydberg state at the two-photon level. Among the decay processes of these superexcited states, dissociation to electronically excited S atoms is dominant as compared to autoionization to ionic states S2+ (X2Πg) at wavelengths λ < 288 nm. The anisotropy parameter extracted from these images reflects the parallel character of these electronic transitions. In contrast, autoionization is found to be particularly efficient at S(1D) and S(1S) detection wavelengths around 288 nm. Information obtained from the kinetic energy distributions of S atoms has revealed the existence of vibrationally excited S2+ (X2Πg (v+ > 11)) that dissociates to ionic products following one-photon absorption. This work also reveals many interesting features of S2 photodynamics compared to those of electronically analogous O2.

Photoionization cross sections measurements of the excited states of lutetium and ytterbium in the near threshold region.

In this work, the threshold photoionization cross sections from the excited states of lutetium and ytterbium atoms were investigated by the laser pump-probe scheme under the condition of saturated resonant excitation. We obtained the resonance enhanced multiphoton ionization spectra of the lutetium and ytterbium atoms of the lanthanide metals in the range of 307.50-312.50 nm and 265.00-269.00 nm, respectively; the photoionization cross sections of the 5d6s(1D)6p(2D05/2) and 5d6s(3D)6p(2P01/2) states of lutetium and the 4f13(2F0)5d6s2(J = 1) states of ytterbium above threshold regions (0.4-1.6 eV) were measured, and measured values ranged from 2.3 ± 0.2 to 17.7 ± 1.5 Mb.

Probing the Charge-Transfer Potential Energy Surfaces by the Photodissociation of [Ar-N2].

Chemical reaction pathways and product state correlations of gas-phase ion-molecule reactions are governed by the involved potential energy surfaces (PESs). Here, we report the photodissociation dynamics of charge-transfer complex [Ar-N2]+, which is the intermediate of the model system of the Ar+ + N2 → Ar + N2+ reaction. High-resolution recoiling velocity images of photofragmented N2 and N2+ from different dissociation channels exhibit a vibrational state-specific correlation, revealing the nonadiabatic charge-transfer mechanisms upon the photodissociation of [Ar-N2]+. The state-resolved product branching ratios have yielded an accurate determination of the resonant charge-transfer probabilities. This work provides a powerful approach to elucidating the detailed dynamics of chemical events of charge-transfer complex [Ar-N2]+ and to probing the state-to-state charge-transfer PESs.

Dissociation dynamics of carbon dioxide cation (CO2 +) in the C2Σg + state via [1+1] two-photon excitation.

The dissociation dynamics of CO2 + in the C2Σg + state has been studied in the 8.14-8.68 eV region by [1+1] two-photon excitation via vibronically selected intermediate A2Πu and B2Σu + states using a cryogenic ion trap velocity map imaging spectrometer. The cryogenic ion trap produces an internally cold mass selected ion sample of CO2 +. Total translational energy release (TER) and two-dimensional recoiling velocity distributions of fragmented CO+ ions are measured by time-sliced velocity map imaging. High resolution TER spectra allow us to identify and assign three dissociation channels of CO2 + (C2Σg +) in the studied energy region: (1) production of CO+(X2Σ+) + O(3P) by predissociation via spin-orbit coupling with the repulsive 14Πu state; (2) production of CO+(X2Σ+) + O(1D) by predissociation via bending and/or anti-symmetric stretching mediated conical intersection crossing with A2Πu or B2Σu +, where the C2Σg +/A2Πu crossing is considered to be more likely; (3) direct dissociation to CO+(A2Π) + O(3P) on the C2Σg + state surface, which exhibits a competitive intensity above its dissociation limit (8.20 eV). For the first dissociation channel, the fragmented CO+(X2Σ+) ions are found to have widely spread populations of both rotational and vibrational levels, indicating that bending of the parent CO2 + over a broad range is involved upon dissociation, while for the latter two channels, the produced CO+(X2Σ+) and CO+(A2Π) ions have relatively narrow rotational populations. The anisotropy parameters ß are also measured for all three channels and are found to be nearly independent of the vibronically selected intermediate states, likely due to complicated intramolecular interactions in the studied energy region.

N-loss photodissociation dynamics of N2O+(B2Π) near the NO+(Σ+1) + N(2P) dissociation limit.

Photodissociation dynamics of the N2O+ cation in its B2Π state has been experimentally studied in an energy region around the NO+(1Σ+) + N(2P) dissociation limit using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The results show that the NO+(1Σ+) + N(2D) product channel dominates the dissociation dynamics and requires the NNO angle to change by 30°-50° prior to dissociation. The NO+(1Σ+) + N(2P) product channel, which directly correlates with the B2Π state but less competitive, opens immediately when the photon energy reaches the dissociation limits, indicating a flat dissociation pathway without bending on the B2Π state surface.

A2Πu and 14Σu- States of Br2+ Studied by [1+1] Two-Photon Dissociation Spectroscopy in a Cold Ion Beam.

The A2Πu-X2Πg and 14Σu--X2Πg electronic transition spectra of Br2+ have been studied in the 500-720 nm wavelength range in a cold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The cryogenic ion trap produces a rotationally and vibrationally cold mass selected ion beam of Br2+, which simplifies the experimental spectra from vibrational hot bands and bands of mixed isotopic species. Vibrationally resolved photofragment excitation spectra are recorded for individual isotopologues of Br2+ (79Br2+, 79Br81Br+, 81Br2+) by [1+1] two-photon dissociation spectroscopy. Velocity map imaging of the photofragmented Br+ ions provides complementary information in the determination of spin-orbit states involved in corresponding electronic transitions. An experimental identification of the 14Σu- state has becomes possible based on the present experimental results and previously reported theoretical calculations. Vibrational analyses of the photofragment excitation spectra have yielded spectroscopic parameters, including state origins, harmonic frequencies, and anharmonic constants, for both A2Πu and 14Σu- states. The observed A2Πu state spin-orbit splitting and the "spin-forbidden" 14Σu--X2Πg transition band intensities indicate considerable spin-orbit couplings between the 14Σu- and A2Πu states. In addition, two groups of weak vibrational bands are also observed in the experimental spectra of 79Br81Br+, which may be due to symmetry-forbidden transitions from the X2Πg ground state to low-lying gerade states.

A cryogenic cylindrical ion trap velocity map imaging spectrometer.

A cryogenic cylindrical ion trap velocity map imaging spectrometer has been developed to study photodissociation spectroscopy and dynamics of gaseous molecular ions and ionic complexes. A cylindrical ion trap made of oxygen-free copper is cryogenically cooled down to ∼7 K by using a closed cycle helium refrigerator and is coupled to a velocity map imaging (VMI) spectrometer. The cold trap is used to cool down the internal temperature of mass selected ions and to reduce the velocity spread of ions after extraction from the trap. For CO2 + ions, a rotational temperature of ∼12 K is estimated from the recorded [1 + 1] two-photon dissociation spectrum, and populations in spin-orbit excited X2Πg,1/2 and vibrationally excited states of CO2 + are found to be non-detectable, indicating an efficient internal cooling of the trapped ions. Based on the time-of-flight peak profile and the image of N3 +, the velocity spread of the ions extracted from the trap, both radially and axially, is interpreted as approximately ±25 m/s. An experimental image of fragmented Ar+ from 307 nm photodissociation of Ar2 + shows that, benefitting from the well-confined velocity spread of the cold Ar2 + ions, a VMI resolution of Δv/v ∼ 2.2% has been obtained. The current instrument resolution is mainly limited by the residual radial speed spread of the parent ions after extraction from the trap.

Photodissociation dynamics of OCS at ∼210 nm: The role of c(23A″) state.

Photodissociation dynamics of carbonyl sulfide (OCS) in the deep ultraviolet region is investigated using a time-sliced ion velocity map imaging technique. The measured total kinetic energy release spectra from the photodissociation of OCS at ∼210 nm shows three dissociation channels to the fragment S(1D2), corresponding to low, medium, and high kinetic energy release (ET), respectively. The high ET channel is found to be a new dissociation channel opening with photolysis wavelength at ∼210 nm. Based on the aq(k)(p) polarization parameters as well as the anisotropy parameters ß determined from the images of S(1D2), the dissociation of OCS to S(1D2) + CO at 210 nm is concluded to involve a direct vertical excitation of the triplet c(23A″) state from the ground state, followed by processes as: the low ET component arises from a non-adiabatic transition from the repulsive A(21A') state to the electronic ground state X(11A'); the medium ET component arises from a simultaneous excitation to two repulsive excited states; and the high ET component arises from the intersystem crossing from the triplet c(23A″) state to the repulsive A(21A') state. The present study shows that, due to the strong spin-orbit coupling between the triplet c(23A″) state and the repulsive A(21A') state, a direct excitation to c(23A″) significantly contributes to the photodissociation dynamics of OCS in the deep-UV region.