Multiphoton Ionization Method Research Articles

Spectroscopic Study of a New Electronic Band System 33Δg-a3Πu of C2.

As one of the most important diatomic molecules in the universe, the spectroscopic characterizations of C2 have attracted wide attention in various fields, such as interstellar chemistry, planetary atmospheric chemistry, and combustion. In recent years, a systematic spectroscopic study of C2 in the vacuum ultraviolet (VUV) region has been carried out in our laboratory by using the (1VUV+1'UV) resonance-enhanced multiphoton ionization method based on the combination of a tunable VUV laser source and a time-of-flight mass spectrometer. Two new electronic transition band systems have been reported, following the pioneering work of Herzberg and co-workers in 1969. In the current study, a total of 18 vibronic transition bands of C2 from the lower a3Πu state are experimentally observed in the VUV photon energy range 72000-81000 cm-1, and 6 new upper vibronic levels of 3Δg symmetry are identified, which are assigned as the v' = 0-5 vibrational levels of the 33Δg state of C2. The term energy Te of the 33Δg state is determined to be in the range of 78425-78475 cm-1 (9.724-9.730 eV) with respect to the ground X1Σg+ state, and the molecular constants such as vibrational and rotational constants are also determined, which are in reasonable agreement with those predicted by high-level ab initio theoretical calculations. Irregular vibrational energy level spacings in the 33Δg state are observed, which is tentatively attributed to the strong perturbations between the 33Δg and 23Δg states, as previously predicted by theory.

Direct Determination of the Activation Energy for Diffusion of OH Radicals on Water Ice

Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods, the behaviors of OH radicals on the surface of an interstellar ice analog were monitored at temperatures between 54 and 80 K. The OH number density on the surface of ultraviolet-irradiated compact amorphous solid water gradually decreased at temperatures above 60 K. Analyzing the temperature dependence of OH intensities with the Arrhenius equation, the decrease can be explained by the recombination of two OH radicals, which is rate-limited by thermal diffusion of OH. The activation energy for surface diffusion was experimentally determined for the first time to be 0.14 ± 0.01 eV, which is larger than or equivalent to those assumed in theoretical models. This value implies that the diffusive reaction of OH radicals starts to be activated at approximately 36 K on interstellar ice.

Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogs: Diamond-like Carbon and Graphite

Abstract Hydrogen molecules have two nuclear spin isomers: ortho-H2 and para-H2. The ortho-to-para ratio (OPR) is known to affect chemical evolution as well as gas dynamics in space. Therefore, understanding the mechanism of OPR variation in astrophysical environments is important. In this work, the nuclear spin conversion (NSC) processes of H2 molecules on diamond-like carbon and graphite surfaces are investigated experimentally by employing temperature-programmed desorption and resonance-enhanced multiphoton ionization methods. For the diamond-like carbon surface, the NSC time constants were determined at temperatures of 10–18 K and from 3900 ± 800 s at 10 K to 750 ± 40 s at 18 K. Similar NSC time constants and temperature dependence were observed for a graphite surface, indicating that bonding motifs (sp3 or sp2 hybridization) have little effect on the NSC rates.

Experimental Approach to Physicochemical Hydrogen Processes on Cosmic Ice Dust

Hydrogen is the most abundant element in the universe, and thus atomic and molecular hydrogen play important roles in chemical evolution in space. In particular, the physicochemical processes of hydrogen on cosmic dust, such as the diffusion of H atoms and nuclear spin conversion of H2 molecules at low temperatures, are recognized to significantly influence the subsequent chemical evolution. However, it is not easy to track the hydrogen on the surface by conventional experiments. We have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect hydrogen on cosmic dust analogues. In this paper, we present a brief review of our recent experiments for elucidating the behavior of hydrogen on water ice, pure solid carbon monoxide, and diamond-like carbon as cosmic dust analogue surfaces.

Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption

Abstract The interactions of atomic and molecular hydrogen with bare interstellar dust grain surfaces are important for understanding H2 formation at relatively high temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and the desorption energetics of H2 molecules on an amorphous diamond-like carbon (DLC) surface. From temperature-programmed desorption experiments with a resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the H2 coverage-dependent activation energies for H2 desorption are determined. The activation energies decrease with increasing H2 coverage and are centered at 30 meV with a narrow distribution. Using a combination of photostimulated desorption and REMPI methods, the time variations of the surface number density of H2 following atomic and molecular hydrogen depositions are studied. From these measurements, we show that H2 formation on a DLC surface is quite efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2 recombination reactions suggests that H-atom diffusion on a DLC surface is mediated by quantum mechanical tunneling. In astrophysically relevant conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at 20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at relatively high temperatures.

Behaviour of radicals on interstellar dust analogues

AbstractSurface reactions of radicals play important roles in the formation of complex molecules on interstellar dust grains. Under interstellar conditions, because the coverage of adsorbates on dust is significantly low, surface reactions are often limited by precedent processes, namely, the adsorption and diffusion of reactants. Therefore, to appropriately incorporate dust surface reactions into chemical models, information on the adsorption and diffusion of radicals is crucial. However, it is not easy to follow the behaviour of radicals on surfaces by conventional experimental methods. To monitor radicals on interstellar dust analogues, we have recently succeeded in applying a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods. In this paper, we briefly review our recent experiments for clarifying radical behaviour on water ice, pure solid CO and diamond-like carbon.

Measurements of the Activation Energies for Atomic Hydrogen Diffusion on Pure Solid CO

Abstract The diffusion of hydrogen atoms on dust grains is a key process in the formation of interstellar H2 and some hydrogenated molecules such as formaldehyde and methanol. We investigate the adsorption and diffusion of H atoms on pure solid CO as an analog of dust surfaces observed toward some cold interstellar regions. Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect H atoms directly, the relative adsorption probabilities and diffusion coefficients of the H atoms are measured on pure solid CO at 8, 12, and 15 K. There is little difference between the diffusion coefficients of the hydrogen and deuterium atoms, indicating that the diffusion is limited by thermal hopping. The activation energies controlling the H-atom diffusion depend on the surface temperature, and values of 22, 30, and ∼37 meV were obtained for 8, 12, and 15 K, respectively.

Spectroscopic characterization on the neutral excited state and cationic ground state of styrene

The spectroscopic properties of styrene have been investigated via resonance-enhanced multiphoton ionization (REMPI) and slow electron velocity-map imaging (SEVI) methods. Vibrational frequencies in the first excited state of neutral (S1) and ground electronic state of cation (D0) have been accurately measured. From the one-color REMPI spectrum of styrene, the S1⟵S0 electron transition energy is determined as 34729±3cm−1. Most low-frequency vibrations of S1 state are unambiguously identified with the aid of calculated results at TD-B3LYP/6–311++G(d, p) level. Some controversially high-frequency vibrational modes of S1 state are reassigned in comparison with theoretical results. The SEVI spectra taken via various S1 intermediate levels provide the detailed vibrational structures of D0 state and directly yield the accurate adiabatic ionization potential (IP) of 68267±6cm−1. The vibrational frequencies of the D0 state calculated at the B3LYP/6–311++G(d, p) level are in excellent agreement with the experimental results to aid us assigning some high-frequency vibrational modes.

Infrared laser induced population transfer and parity selection in (14)NH3: A proof of principle experiment towards detecting parity violation in chiral molecules.

We have set up an experiment for the efficient population transfer by a sequential two photon-absorption and stimulated emission-process in a molecular beam to prepare quantum states of well defined parity and their subsequent sensitive detection. This provides a proof of principle for an experiment which would allow for parity selection and measurement of the time evolution of parity in chiral molecules, resulting in a measurement of the parity violating energy difference ΔpvE between enantiomers of chiral molecules. Here, we present first results on a simple achiral molecule demonstrating efficient population transfer (about 80% on the average for each step) and unperturbed persistence of a selected excited parity level over flight times of about 1.3 ms in the beam. In agreement with model calculations with and without including nuclear hyperfine structure, efficient population transfer can be achieved by a rather simple implementation of the rapid adiabatic passage method of Reuss and coworkers and considering also the stimulated Raman adiabatic passage technique of Bergmann and coworkers as an alternative. The preparation step uses two powerful single mode continuous wave optical parametric oscillators of high frequency stability and accuracy. The detection uses a sensitive resonantly enhanced multiphoton ionization method after free flight lengths of up to 0.8 m in the molecular beam. Using this technique, we were able to also resolve the nuclear hyperfine structure in the rovibrational levels of the ν1 and ν3 fundamentals as well as the 2ν4 overtone of (14)NH3, for which no previous data with hyperfine resolution were available. We present our new results on the quadrupole coupling constants for the ν1, ν3, and 2ν4 levels in the context of previously known data for ν2 and its overtone, as well as ν4, and the ground state. Thus, now, (14)N quadrupole coupling constants for all fundamentals and some overtones of (14)NH3 are known and can be used for further theoretical analysis.

Signatures of Quantum-Tunneling Diffusion of Hydrogen Atoms on Water Ice at 10 K.

Reported here is the first observation of the tunneling surface diffusion of a hydrogen (H) atom on water ice. Photostimulated desorption and resonance-enhanced multiphoton ionization methods were used to determine the diffusion rates at 10 K on amorphous solid water and polycrystalline ice. H-atom diffusion on polycrystalline ice was 2 orders of magnitude faster than that of deuterium atoms, indicating the occurrence of tunneling diffusion. Whether diffusion is by tunneling or thermal hopping also depends on the diffusion length of the atoms and the morphology of the surface. Our findings contribute to a better understanding of elementary physicochemical processes of hydrogen on cosmic ice dust.

Vector correlations in the F + HD reaction

A theoretical study of the orientation of product rotational angular momenta for two chemical reaction channels: F + HD(ν r = 0, j r = 0) → HF(ν, j) + D and F + HD(ν r = 0, j r = 0) → DF(ν, j) + H at a E coll = 78.54 meV collision energy was performed. Angular momentum orientation was described on the basis of irreducible tensor operators (state multipoles) expressed through anisotropy transfer coefficients, which contained quantum-mechanical scattering T matrices determined on the basis of exact solutions to quantum scattering equations obtained using the hyperquantization algorithm. The possibility of the existence of substantial orientation of the angular momentum of reaction products j in the direction perpendicular to the scattering plane was demonstrated. The dependences of differential reaction cross sections and state multi-poles on the ν and j quantum numbers were calculated and analyzed. A experimental scheme based on the multiphoton ionization method was suggested. The scheme can be used to detect predicted reaction product angular momentum orientation.

The study of ungerade electronic states of the Xe2 molecules in the region of Xe*(5p 56p, 5d, 7s, 6d) by the resonance multiphoton ionization method

Electronic spectra of the Xe2 molecules in the energy range of 77700–89300 cm−1 are recorded. The method of resonance enhanced multiphoton ionization of molecules in a supersonic molecular beam was used, in which excitation of the molecules by three photons was followed by ionization caused by a fourth photon (the (3+1) REMPI method). Analysis of the vibrational structure of observed systems of bands yielded information about the dissociation energy and the molecular constants for ungerade states of molecules. On the basis of the Franck-Condon principle, the equilibrium distances for potential curves were estimated from the relative intensities in vibrational progressions. Data on 16 new electronic states of diatomic xenon molecules with the dissociation limits Xe 2 * → XE(5p 6 1 S 0) + Xe*(5p 56p,5d, 7s, 7p) were obtained.

Mode specific photodissociation of CS2+via the A2Πu state: a time-sliced velocity map imaging study

The vibrationally mediated photodissociation of CS(2)(+) cations via the A(2)Π(u)(ν(1),ν(2),0) state has been studied by means of the velocity map ion imaging technique. The measurements were made with a double resonance strategy. The CS(2)(+) cations were prepared by a (3 + 1) resonance enhanced multiphoton ionization method. The photo-fragment excitation spectrum of S(+) was recorded by scanning the photolysis laser via the A(2)Π(u)(ν(1),ν(2),0) state. By fixing the photolysis laser wavelength at the specific vibrational state, the (1 + 1) photodissociation images of S(+) photofragments from numerous vibrationally mediated states have been accumulated. The translational energy release spectra derived from the resulting images imply that the co-fragments, CS radicals, are both vibrationally and rotationally excited. The one-photon photodissociation without the vibrational state selection has also been performed. Comparing the vibrationally mediated photodissociation with one-photon photodissociation observations, clear evidence of vibrational state control of the photodissociation process is observed.

Femtosecond Multiphoton Ionization of Pyrrole

The photoionization dynamics of pyrrole are investigated by using a photoelectron imaging method and a tunable femtosecond laser. Two-photon nonresonant ionization experiments in the wavelength range from 261 to 298 nm indicate that the cation and neutral ground states have similar structures. The main vibrational excitation in the cation ground state is the v(8) mode. Two-photon absorption at 406 nm projects neutral pyrrole into a mixed state comprising the 1B(2) valence and 3p Rydberg states. Ionization from this mixed state mainly results in the overtone excitation of vibrational mode v(8) and v(9) of the cation state. In the wavelength range from 336 to 364 nm, a mixed state comprising the 3d/4s Rydberg and the 4A(1) valence states are populated by the absorption of two photons through vibronic coupling. The partition ratio among these states varies with the excitation wavelength, resulting in dramatic changes in both kinetic energy distributions and angular distributions. As the laser wavelength becomes shorter, from 336 to 314 nm, higher excited states, 3B(2), 5A(1), 6A(1), 7B(1) and 4B(2), can be populated. Photoelectron angular distributions provide supplementary verification of assignments. Our experiments indicate that femtosecond multiphoton ionization and photoelectron imaging methods are powerful tools for investigating short-lived intermediated excited states, which cannot be detected in nanosecond experiments.

Electronic spectra of ArXe molecules in the region of Xe* (6s′, 6p, 5d), 77 000–80 200 cm−1, using resonantly enhanced multiphoton ionization

The excited electronic states of ArXe molecules in the region 77 000–80 200 cm−1 were studied using the (2+1) and (3+1) resonance-enhanced multiphoton ionization methods. The use of different methods of multi-photon excitation and Ar+ ion registration allowed us to obtain some new data. Molecular constants were obtained for previously unknown excited states of molecules with the following dissociation limits: ArXe* → Ar1S0+Xe*6р[5/2]3 with Ω = 2, 3 symmetry; Ar1S0+Xe*6p[3/2]2 with Ω = 1, 2 symmetry; Xe0S1 → Xe*6s′[1/2]01 with Ω = 0+ symmetry.

Formation mechanisms of oxygen atoms in the O(PJ3) state from the 157nm photoirradiation of amorphous water ice at 90K

Desorption of ground state O((3)P(J=2,1,0)) atoms following the vacuum ultraviolet photolysis of water ice in the first absorption band was directly measured with resonance-enhanced multiphoton ionization (REMPI) method. Based on their translational energy distributions and evolution behavior, two different formation mechanisms are proposed: One is exothermic recombination reaction of OH radicals, OH+OH-->H(2)O+O((3)P(J)) and the other is the photodissociation of OH radicals on the surface of amorphous solid water. The translational and internal energy distributions of OH radicals as well as the evolution behavior were also measured by REMPI to elucidate the roles of H(2)O(2) and OH in the O((3)P(J)) formation mechanisms.

Formation mechanisms of oxygen atoms in the O(D21) state from the 157nm photoirradiation of amorphous water ice at 90K

Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O((1)D) and OH(v=0,1), were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O((1)D) atoms from the ice surface. One is unimolecular dissociation of H(2)O to H(2)+O((1)D) as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H(2)O(2) is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice.

Desorption of hydroxyl radicals in the vacuum ultraviolet photolysis of amorphous solid water at 90 K

We have studied the desorption dynamics of OH radicals from the 157 nm photodissociation of amorphous solid water (ASW) as well as H(2)O(2) deposited on an ASW surface at 90 K. The translational and internal energy distributions of OH were measured using resonance-enhanced multiphoton ionization methods. These distributions are compared to reported molecular dynamics calculations for the condensed phase photodissociation of water ice and also reported results for the gas phase photodissociation of H(2)O at 157 nm. We have confirmed that OH radicals are produced from two different mechanisms: one from primary photolysis of surface H(2)O of ASW, and the other being secondary photolysis of H(2)O(2) photoproducts on the ASW surface after prolonged irradiation at 157 nm.

Time-resolved measurements of low concentration aromatic hydrocarbons in diesel exhaust using a resonance enhanced multi-photon ionization method

Time-resolved measurements of the concentration of volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) in exhaust from a diesel vehicle fitted with an oxidation catalyst and operated in JE05 mode are performed at a sensitivity of 10 ppb using a supersonic jet/resonance enhanced multi-photon ionization (Jet-REMPI) method. The concentrations of benzene, naphthalene, and phenol in exhaust from the test vehicle are measured before entering and after exiting from the oxidation catalyst. The total hydrocarbon (THC) is measured simultaneously using a constant volume sample (CVS) instrument fitted with a flame ionization detector (FID). Concentration changes of benzene, naphthalene and phenol are recorded at 1 s intervals and quantified using standard samples. Comparison of these signals with the real-time THC data shows that the time dependence of the individual species is almost the same as that of the THC before the oxidation catalyst but substantially different after.

Ab initio post-HF study of guanine dimer isomers involving imino forms in gas phase

We have optimized some guanine dimer (GG) isomers including imino forms with MP2/6-31++G(d,p) level. Contrary to the Hartree–Fock level of calculation, our optimized GG structures are found to be slightly non-planar due to the electron correlation effect. Though the interaction energy of the GG isomer including imino forms is slightly greater than that of keto form dimer, our calculated vibrational frequencies for GG isomers including imino forms are not in reasonable agreement with the spectra of the resonance enhanced multiphoton ionization (REMPI) method by Nir et al. Our MP2 results suggest that the species observed by their REMPI method should be keto form dimer.