Predissociation Process Research Articles

Strongly Rotationally Dependent Lifetimes of C2 in the 23Σg- State: Implications for Predissociation in the Vacuum Ultraviolet Region.

The radiative and photodissociative properties of the dicarbon molecule, C2, in high-lying electronic states are of utmost importance for modeling the photochemical processes that occur in various astronomical environments. Despite extensive spectroscopic studies in the last two centuries, the photodissociation properties of C2 are still largely unknown, particularly for quantum states in the vacuum ultraviolet (VUV) region. Here, the lifetimes of C2 for each individual rovibrational level in the recently identified 23Σg- state are measured for the first time using a VUV-pump-UV-probe photoionization scheme. The lifetimes are found to be strongly dependent on the rotational and vibrational quantum levels in the 23Σg- state. The strongly rotationally dependent lifetimes observed here indicate that the 23Σg- state may mainly undergo a predissociation process through couplings with nearby repulsive electronic states. The current observation could have important applications in modeling the interstellar medium and cometary comae.

Ion-core switching in Rydberg series of XeKr

We investigate XeKr in high Rydberg states by mass-resolved optical-optical double resonance excitation spectroscopy. We excite XeKr via two-photon absorption to the v* = 0 and v* = 2 vibrational levels of an intermediate electronically excited state of Xe*Kr correlated to Xe*6p[5/2]2 and excite Xe*Kr further via one-photon absorption to the high Rydberg states of Xe**Kr in the energy range of 93200–97400 cm−1. The potential energy curves and the quantum defects of the d-Rydberg and s-Rydberg series of Xe**Kr, converging the A 2Π3/2 state of XeKr+ are determined. From the kinetic energy release in the predissociation process, we conclude that the ion-core switching occurs by the interaction between the bound electronic states of Xe**Kr converging to the A 2Π3/2 state of XeKr+ and the repulsive electronic states of low-lying electronic states of XeKr**. Based on numerical simulations, we interpret the dependence of the yield of the ion-core switching on the excitation energy in terms of the population transfer from the bound electronic states of the high Rydberg states Xe**Kr, converging to the A 2Π3/2 state of XeKr+, to those converging to the electronic ground X2 Σ 1 / 2 + state of XeKr + .

Dynamics of the H2 levels lying between the third and fourth dissociation thresholds (132 500 – 139 000 cm− 1)

The absorption spectrum of H2 at room temperature, was recorded at high resolution at the BESSY II synchrotron radiation storage ring in the 132500 – 139000 cm−1 energy range, allowing the study of the dynamics of the excited levels (npσ 1Σu+ and npπ 1Πu±) located between the H(n = 3) + H(n = 1) and H(n = 4) + H(n = 1) thresholds. Ionization anddissociation were found to compete for levels with n ≤ 8. In most cases, for the levels with n > 8, ionization is the only decay channel. The relative importance of the direct ionization, direct dissociation, autoionization and predissociation processes was evaluated for the whole energy range. From the different detected channels, and from the comparison with theoretical approaches, the main continua responsible for the predissociations were assigned to the V and to the B"B¯ states and the H(3p)/H(n = 3) ratios of the dissociation fragments from the 4pπ and 5pπ 1Πu- levels were measured. Molecular fluorescence was observed for few 1Πu- levels and the importance of its contribution is discussed.

Reinvestigation of the Herzberg-Lagerqvist-Malmberg transitions of C2 in vacuum ultraviolet region and the implications for astronomical observations.

The dicarbon radical, C2, is one of the most abundant molecules in the universe, and has been widely observed in various energetic environments. Even though numerous experimental and theoretical investigations on C2 have been done during the last two centuries, spectroscopic study of C2 in vacuum ultraviolet (VUV) region has been rare. The only three known absorption band systems in VUV region were identified by Herzberg and co-workers in 1969 by VUV spectrograph, namely the electronic transitions F1Πu(v')-X1Σg +(v″), f3Σg -(v')-a3Πu(v″) and g3Δg(v')-a3Πu(v″) (Herzberg-Lagerqvist-Malmberg transitions). In this study, we employ a two-photon resonance-enhanced four-wave mixing based VUV laser source and a time-of-flight mass spectrometer for reinvestigating the above three electronic transitions of C2 through a resonant (1VUV + 1'UV) photoionization scheme. Besides those vibronic transitions as identified by Herzberg and co-workers, many more absorption bands belonging to the electronic transitions f3Σg -(v')-a3Πu(v″) and g3Δg(v')-a3Πu(v″) are observed with their spectroscopic parameters determined. The rather astrophysically important F1Πu state is not observed here by the resonant (1VUV + 1'UV) photoionization scheme, which must be due to its fast predissociation process. Instead, our study shows that the vibronic band g3Δg(v' = 2)-a3Πu(v″ = 0) exactly overlaps with F1Πu(v' = 0)-X1Σg +(v″ = 0), which was not realized in previous studies. The potential implications of these findings to astronomical observations are discussed.

Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion.

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, β, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, β > 0 for the P-branch excitations, while β < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.

Photodissociation dynamics of CS2 near 204 nm: The S(3PJ)+CS(X1Σ+) channels

We study the photodissociation dynamics of CS2 in the ultraviolet region using the time-sliced velocity map ion imaging technique. The S(3PJ)+CS(X1Σ+) product channels were observed and identified at four wavelengths of 201.36, 203.10, 204.85 and 206.61 nm. In the measured images of S(3PJ=2,1,0), the vibrational states of the CS(X1Σ+) co-products were partially resolved and the vibrational state distributions were determined. Moreover, the product total kinetic energy releases and the anisotropic parameters were derived. The relatively small anisotropic parameter values indicate that the S(3PJ=2,1,0)+CS(X1Σ+) channels are very likely formed via the indirect predissociation process of CS2. The study of the S(3PJ=2,1,0)+CS(X1Σ+) channels, which come from the spin-orbit coupling dissociation process of CS2, shows that nonadiabatic process plays a role in the ultraviolet photodissociation of CS2.

Coupled nuclear-electronic decay dynamics of O2 inner valence excited states revealed by attosecond XUV wave-mixing spectroscopy.

Multiple Rydberg series converging to the O2+c4Σ-u state, accessed by 20-25 eV extreme ultraviolet (XUV) light, serve as important model systems for the competition between nuclear dissociation and electronic autoionization. The dynamics of the lowest member of these series, the 3sσg state around 21 eV, has been challenging to study owing to its ultra-short lifetime (<10 fs). Here, we apply transient wave-mixing spectroscopy with an attosecond XUV pulse to investigate the decay dynamics of this electronic state. Lifetimes of 5.8 ± 0.5 fs and 4.5 ± 0.7 fs at 95% confidence intervals are obtained for v = 0 and v = 1 vibrational levels of the 3s Rydberg state, respectively. A theoretical treatment of predissociation and electronic autoionization finds that these lifetimes are dominated by electronic autoionization. The strong dependence of the electronic autoionization rate on the internuclear distance because of two ionic decay channels that cross the 3s Rydberg state results in the different lifetimes of the two vibrational levels. The calculated lifetimes are highly sensitive to the location of the 3s potential with respect to the decay channels; by slight adjustment of the location, values of 6.2 and 5.0 fs are obtained computationally for the v = 0 and v = 1 levels, respectively, in good agreement with experiment. Overall, an intriguing picture of the coupled nuclear-electronic dynamics is revealed by attosecond XUV wave-mixing spectroscopy, indicating that the decay dynamics are not a simple competition between isolated autoionization and predissociation processes.

Radiative cooling dynamics of isolated N2O+ ions in a cryogenic electrostatic ion storage ring

We study radiative cooling processes of ${\mathrm{N}}_{2}{\mathrm{O}}^{+}$ ions isolated in vacuum by taking advantage of the extended storage times possible with a cryogenic electrostatic ion storage ring. The temporal evolution of the internal state populations of the ${\mathrm{N}}_{2}{\mathrm{O}}^{+}$ ions after being stored in the ring was state-selectively measured by action spectroscopy via a predissociation process using a wavelength-tunable pulsed laser. The rotational level population remained unchanged up to about 200 s after ion storage, which is consistent with theoretical predictions and the small permanent dipole moment. On the other hand, we found that depletion of the vibrational excited states in the NN-O stretching mode proceeded within 5 s, while a noticeable increase of the ground-state population was not observed. This implies that the initial populations of the vibrational excited states are relatively small. We discuss such behavior in the context of the initial population distribution of ions extracted from the ion source with a simulation based on simple cascade modeling.

Vibrational and rotational cooling dynamics of the triatomic molecular ion N2O+ stored in RICE

SynopsisWe studied radiative cooling dynamics of the triatomic molecular ion N2O+ stored in RICE. We observed the evolution of the populations at vibrational and rotational levels up to several 100 s by measuring neutral fragments from the laser-induced predissociation process. We found clear evidence of radiative cooling of the vibrational level populations with a time constant of several seconds. In contrast, absence of significant changes in the rotational levels revealed that the time scale of the rotational cooling is far longer, which is consistent to our simulation.

Branching Ratios of the N(2D03/2) and N(2D05/2) Spin-Orbit States Produced in the State-Selected Photodissociation of N2 Determined Using Time-Sliced Velocity-Mapped-Imaging Photoionization Mass Spectrometry (TS-VMI-PI-MS).

Branching ratios for N(2D03/2) and N(2D05/2) produced by predissociation of state selected excited nitrogen molecules in the vacuum ultraviolet region have been measured for the first time. The quantum numbers of the excited nitrogen molecule are defined by selective excitation of the nitrogen molecule in the Franck-Condon region from the ground electronic, 1Σg+, vibrational, v″, and rotational, J″ state to an excited Eu', v', J' state with a tunable vacuum ultraviolet, VUV1, laser. The neutral atoms produced by predissociation from this excited state are then selectively ionized with a second tunable VUV2 laser. Measurement of the relative populations of these two atoms formed in their spin-orbit states defines the quantum states for the atomic products. This means that the wave functions of the initial state and knowledge of the relative yields define all the experimental parameters for this series of unimolecular reactions. The ions formed by VUV2 are mass analyzed with a time-of-flight mass spectrometer and detected with a time slice velocity ion imaging mass spectrometer. In this manner, we can determine the recoil velocity associated with the predissociation process. Two different techniques are used to determine the spin-orbit ratios, namely, resonant VUV photoionization (RVUV-PI) spectroscopy and total kinetic energy release (TKER) spectroscopy determined from the image produced when the atoms are selectively ionized by VUV2 in the interaction region. The TKER spectra obtained from the lines at 110 296.25 and 110 304.96 cm-1 that couple to a newly discovered autoionization line at 129 529.4255 ± 0.0015 cm-1 prove that the lines observed in this region originate from the N(2D03/2) and N(2D05/2) atoms. Two other lines in this region at 110 286.20 and 110 299.89 cm-1 originate from the nitrogen N(4S03/2) that is photoionized in a 1+ 1 VUV-UV resonant multiphoton ionization process. The spin-orbit branching ratios have been evaluated for valence and Rydberg electronic excited states from 104 129.4 to 118 772.1 cm-1, and it shows that they are independent of the rotational and vibrational quantum numbers. They are not appreciably affected by the symmetry properties of the wave function in the Franck-Condon region of the excited states. In the energy region below 117 153.8 cm-1 the pathways at long internuclear distances appear to determine [N(2D03/2)]/[N(2D05/2)] branching ratios of ∼0.38, ∼0.62, and ∼1.04. At higher energies, TKER and RVUV-PI spectroscopy have been used to show that the average fraction of the N(2D03/2) and N(2D05/2) atoms produced in the spin-allowed channels that produce two N(2D0J) is 0.85 versus 0.15 for spin-forbidden channels. The importance and need for this information for comparison with theory and applications in astrochemistry are briefly discussed.

Symmetry-induced kinetic isotope effects in the dissociation dynamics of CHCl3+ and CHCl4−

Loss of Cl from CHCl3+ with little excess energy is found to proceed via two distinct pathways, the reaction channel which is being followed is determined by a symmetry-induced kinetic isotope effect. Computational investigations suggest competition between a statistical adiabatic process and a process of electronic predissociation. For CHCl4− and CH2Cl3−, rotational predissociation contributes to the kinetic isotope effects for all isotopomers, whereas the particularly large effect observed for CH35Cl337Cl− also originates from symmetry-induced degeneracy of vibrational modes in the C3V point.

Carbon and oxygen isotopic fractionation in the products of low-temperature VUV photodissociation of carbon monoxide

The carbon and oxygen isotope fractionations occurring during photodissociation of carbon monoxide (CO) by vacuum ultraviolet photons (91–107 nm) at 80 K were measured and the isotopic fractionations due to direct and indirect predissociation processes individually quantified. The isotopic fractionations depend on the photodissociation wavelength. Slope values (δ'17O/δ'18O) in oxygen three-isotope space range from 0.75 to 1.1. The isotopic composition of the products depends on the dissociation dynamics at the upper electronic state (perturbation and coupling associated with that state), which in turn modulates the isotope effect (in this case an enrichment of minor isotopes) inside the gas column due to the saturation of major isotopologue (isotope self-shielding). An explanation in terms of isotope self-shielding would require a quantum yield of one for photodissociation of all isotopologues which is not consistent with the data.

Detection and characterization of the tin dihydride (SnH2 and SnD2) molecule in the gas phase.

The SnH2 and SnD2 molecules have been detected for the first time in the gas phase by laser-induced fluorescence (LIF) and emission spectroscopic techniques through the Ã1B1-X̃1A1 electronic transition. These reactive species were prepared in a pulsed electric discharge jet using (CH3)4Sn or SnH4/SnD4 precursors diluted in high pressure argon. Transitions to the electronic excited state of the jet-cooled molecules were probed with LIF, and the ground state energy levels were measured from single rovibronic level emission spectra. The LIF spectrum of SnD2 afforded sufficient rotational structure to determine the ground and excited state geometries: r0″ = 1.768 Å, θ0″ = 91.0°, r0' = 1.729 Å, θ0' = 122.9°. All of the observed LIF bands show evidence of a rotational-level-dependent predissociation process which rapidly decreases the fluorescence yield and lifetime with increasing rotational angular momentum in each excited vibronic level. This behavior is analogous to that observed in SiH2 and GeH2 and is suggested to lead to the formation of ground state tin atoms and hydrogen molecules.

Multiphoton ionization dissociation dynamics of iodoethane studied with velocity map imaging technique

Halogenated alkanes destroy the ozone layer, and iodoethane is one of the important representative halogenated alkanes. Time-of-flight mass spectrometry and velocity map imaging technique are used for investigating the photoionization dissociation dynamics of iodoethane, induced by 800 nm femtosecond laser. The dissociation mechanisms of iodoethane are obtained and discussed by analyzing the velocity distributions and angular distributions of the fragment ions generated in the dissociation. The measurements by time-of-flight mass spectrometry show that iodoethane cations generates C2H5+, I+, CH2I+, C2H2+, C2H3+ and C2H4+. The fragments related to CI bond fragmentation are C2H5+ ions and I+ ions, and the dissociation mechanisms are C2H5I+ C2H5++I and C2H5I+ C2H5+I+ respectively. Comparison between the configurations before and after ionization shows that the CI bond length is 0.2220 nm before ionization and turns longer and becomes 0.2329 nm after ionization. This indicates that the CI bond becomes more unstable after ionization and is more prone to dissociation. Moreover, the velocity map images of C2H5+ and I+ ions are acquired, from which the speed and angular distribution of C2H5+ and I+ are obtained. The analysis of speed distribution of the fragment ions shows that there are two channels, i.e. high energy channel and low energy channel in the dissociation process for producing C2H5+ and I+ ion. The difference between the ratios of the high energy channel and the low energy channel is small, indicating that the high energy channel and the low energy channel of the two dissociation processes are similar. According to the further analysis of the angular distribution of the fragment ions, it is found that the anisotropy parameter of C2H5+ is close to 0 (isotropic), the production channel of which may correspond to the slow vibration predissociation process. The anisotropy parameters of I+ ions are higher, which may be due to the rapid dissociation process on the repulsive potential energy surface. In addition, the density functional theory is used to calculate the configuration change of the iodoethane molecule before and after ionization, the energy level and oscillator strength for the ionic state in order to obtain more insights into the photodissociation dynamics.

N (2P) production in electron-N2 collisions

A unique detector which is selectively sensitive to low energy metastable atoms, has been used to study the production of ground state N (2P) atoms following collisions of low energy (0–200 eV) electrons with molecular nitrogen. Time-of-flight techniques have revealed the existence of at least two distinct mechanisms yielding this dissociation product. Released kinetic energies in the dissociation have allowed positioning of the parent molecular states in the Franck–Condon region. This, together with excitation probability curves, has allowed probable parent states, such as B′ b′ and C′ 3Πu, to be identified making use of recent theoretical calculations. Both direct and pre-dissociation processes are shown to be involved.

A 1∑+ State Lifetime and Predissociation of CuH

A combined cavity ringdown (CRD) and laser induced fluorescence (LIF) spectroscopic study on the A1∑+−X1∑+ transition of CuH has been presented. The CuH molecule, as well as its deuterated isotopologue CuD, are produced in a supersonic jet expansion by discharging H2 (or D2) and Ar gas mixtures using two copper needles. Different profiles of relative line intensities are observed between the measured LIF and CRD spectra, providing an experimental evidence for the predissociation behavior in the A1∑+ state of CuH. The lifetimes of individual upper rotational levels are measured by LIF, in which the J′-dependent predissociation rates are obtained. Based on the previous theoretical calculations, a predissociation mechanism is concluded due to the strong spin-orbit coupling between the A1∑+ state and the lowest-lying triplet 3∑+ state, and a tunneling effect may also be involved in the predissociation. Similar experiments are also performed for CuD, showing that the A1∑+ state of CuD does not undergo a predissociation process.

Photoelectron diffraction in methane probed via vibrationally resolved inner-valence photoionization cross-section ratios.

Vibrationally resolved photoionization of the 2a1 orbital in methane has been studied both experimentally and theoretically, over a wide range of photon energies (40-475 eV). A vibrational progression associated with the symmetric stretch mode of the 2a1(-1) single-hole state was observed in the experimental photoelectron spectra. Individual vibrational sub-states of the spectra were found to be best modeled by asymmetric line-shapes with linewidths gradually increasing with the vibrational quantum number. This indicates the occurrence of a pre-dissociation process for the involved ionic state, discussed here in detail. Finally, diffraction patterns were observed in the vibrational branching ratios for the first three vibrational sub-states ("v-ratios") of the experimental photoelectron spectra. They are found to be in excellent qualitative agreement with those obtained from ab initio models. Compared with previous studies of the 1a1(-1) core-shell photoionization of methane, the period of oscillation of the v-ratios is found to be very different and the phases are of opposite signs. This suggests a strong interplay between the electron diffraction and interference effects inside the molecular potential.

Identification of a previously unobserved dissociative ionization pathway in time-resolved photospectroscopy of the deuterium molecule.

A femtosecond vacuum ultraviolet (VUV) pulse with high spectral resolution (<200 meV) is selected from the laser-driven high order harmonics. This ultrafast VUV pulse is synchronized with an infrared (IR) laser pulse to study dissociative ionization in deuterium molecules. At a VUV photon energy of 16.95eV, a previously unobserved bond-breaking pathway is found in which the dissociation direction does not follow the IR polarization. We interpret it as corresponding to molecules predissociating into two separated atoms, one of which is photoionized by the following IR pulse. A time resolved study allows us to determine the lifetime of the intermediate predissociation process to be about 1ps. Additionally, the dissociative ionization pathways show high sensitivity to the VUV photon energy. As the VUV photon energy is blueshifted to 17.45eV, the more familiar bond-softening channel is opened to compete with the newly discovered pathway. The interpretation of different pathways is supported by the energy sharing between the electron and nuclei.

Ab-initio potential energy curves of valence and Rydberg electronic states of the PO radical

This paper reports a theoretical study of the electronic states of PO radical. Highly correlated ab initio methods were used for mapping the potential energy curves. Internally contracted multi-reference configuration interaction method with the augmented correlation-consistent basis set (aV5Z) has been employed to carry out the study. After the nuclear motion treatment, the spectroscopic constants and the vibrational energy levels of the doublet and quartet electronic states are determined. The calculated values have been found in a good agreement with the existing experimental and theoretical results. The spin-orbit couplings between interacting states were also determined in the region where the crossings of the potential energy curves occurs. These couplings are capable to induce predissociation processes involving quartet states and producing P and O atoms at their ground and low lying excited electronic states. The theoretical vibrational spectrum was predicted using the Franck Condon factors and the transition moments integrals. The calculated spectrum shows intense peaks involving the Rydberg states in complete accordance with the experiment.

Communication: direct measurements of nascent O((3)P0,1,2) fine-structure distributions and branching ratios of correlated spin-orbit resolved product channels CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2) in VUV photodissociation of CO2.

We present a generally applicable experimental method for the direct measurement of nascent spin-orbit state distributions of atomic photofragments based on the detection of vacuum ultraviolet (VUV)-excited autoionizing-Rydberg (VUV-EAR) states. The incorporation of this VUV-EAR method in the application of the newly established VUV-VUV laser velocity-map-imaging-photoion (VMI-PI) apparatus has made possible the branching ratio measurement for correlated spin-orbit state resolved product channels, CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2), formed by VUV photoexcitation of CO2 to the 4s(10 (1)) Rydberg state at 97,955.7 cm(-1). The total kinetic energy release (TKER) spectra obtained from the O(+) VMI-PI images of O((3)P0,1,2) reveal the formation of correlated CO(ã(3)Π; v = 0-2) with well-resolved v = 0-2 vibrational bands. This observation shows that the dissociation of CO2 to form the spin-allowed CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel has no potential energy barrier. The TKER spectra for the spin-forbidden CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel were found to exhibit broad profiles, indicative of the formation of a broad range of rovibrational states of CO(X̃(1)Σ(+)) with significant vibrational populations for v = 18-26. While the VMI-PI images for the CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel are anisotropic, indicating that the predissociation of CO2 4s(10 (1)) occurs via a near linear configuration in a time scale shorter than the rotational period, the angular distributions for the CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel are close to isotropic, revealing a slower predissociation process, which possibly occurs on a triplet surface via an intersystem crossing mechanism.