Multiphoton Ionization Processes Research Articles

Study on the photodissociation mechanisms of m-bromotoluene at 234 and 267 nm using velocity ion imaging

The Photodissociation dynamics of m-bromotoluene with excitation laser wavelength at 234 and 267 nm has been investigated by velocity ion imaging coupled with resonance-enhanced multiphoton ionization. The speed and angular distributions of Br and Br∗ fragments are determined. Promptly direct dissociation via repulsive a n → σ∗ transition is displayed at 234 nm. At 267 nm, fast pre-dissociation processes via a π → π∗ transition to a singlet state followed by an intersystem cross between the singlet state and a repulsive triplet state is found, and multiphoton dissociative ionization process is also suggested.

Optical-optical double-color and double-resonance multiphoton ionization spectrum of NO2

The optical-optical double resonance multiphoton ionization (OODR-MPI) technique has been applied to the study of the Rydberg states of nitrogen dioxide. The results show that NO2 molecule is ionized though a 1+3+1 double resonance multiphoton ionization process in 605—675nm probe wavelength region. The ionization pathway is: NO2(X2A1)hν1NO2(A2B2)3hν2NO2(3pσu)hν2NO22+e. The OODR-MPI spectrum corresponds to the excitation of NO2 3pσu Rydberg state by a three photon process. The value ω1=(1422.5±23.5) cm-1 and ω2＝(590.5±4.9) cm-1 of symmetric stretching and bending vibration frequencies of this Rydberg state are presented. The position of the band origin and the quantum defect of 3pσu Rydberg state are obtained to be (58331±71) cm-1 and 0.69, respectively.

Predissociation Dynamics of B State of Methyl Iodide with Femtosecond Pump-probe Technique

The predissociation dynamics of B Rydberg state of methyl iodide has been studied with femtosecond two-color pump-probe time-of-flight spectra at pump pulse of 400nm and probe pulse 800 nm. The dominant product channels are the CH3I+ and CH3+ formation. The time-dependent signals for CH3I+ and CH3+ ions are obtained. Both of the signal curves can be fitted by biexponential decays with time constants of O 1 and O 2, O 1 was assigned to the lifetimes of high Rydberg states, which can be accessed by absorbing three 400 nm pump pulses and O 2 reflects the dynamics of B Rydberg state, which is accessed with two pump pulses. The lifetime of B Rydberg state is determined to be about 1.57 ps, which is incredibly consistent with the previous studies. The results were interpreted as a multiphoton dissociative ionization processes.

Coherent Microwave Rayleigh Scattering from Resonance-Enhanced Multiphoton Ionization in Argon

Multiphoton ionization and electron recombination processes are studied in argon using coherent microwave Rayleigh scattering from a localized, resonance-enhanced multiphoton ionization produced plasma. A time dependent one-dimensional plasma dynamic model is developed to predict the time evolution of the microwave scattering from the plasma. Experimental results of the argon ionization spectrum and electron recombination rates are in good agreement with the model predictions.

Rotational Dependence of the Proton Transfer Reaction HBr+ (N+) + CO2 → HOCO+ + Br

Absolute cross sections and rate constants for the endothermic proton transfer reaction of rotationally state selected HBr+ ions with CO2 were measured as a function of the rotational energy for different center-of-mass (c.m.) collision energies. State selection of HBr+ ions was achieved via a (2+1) resonance enhanced multiphoton ionization (REMPI) process. By choosing different pump lines the mean rotational energy of HBr+ was varied from 1.6 to 25.9 meV. All experiments were performed in a linear ion guide apparatus. Collision energies considered ranged from 0.28 to 0.85 eV (c.m.). At the highest collision energies the cross section decreases by about 60% from 10 Å2 to 4 Å2 with increasing rotational energy. In contrast to that, at the lowest collision energies the cross section was about 2.5 Å2, independent of the rotational energy.

Ionization of a single hydrogen-like atom by laser pulse of near-atomic strength

The dynamics of high-harmonic generation and atom ionization by a strong and superstrong laser field are studied. In contrast to many earlier works, the present theory does not impose limitations on the laser field’s strength. We solve the nonrelativistic problem of a single hydrogen-like atom’s ionization from the ground state by a short laser pulse of subatomic, atomic, and superatomic field strength. Within the framework of the proposed method, we investigated the matrix elements of the ionization transition and revealed its substantially nonlinear dependence on the laser field strength. Both ionization and recombination processes are taken into account. The proposed method enables us to take into account the arbitrary order multiphoton ionization processes.

High‐order harmonic generation of heteronuclear diatomic molecules in intense ultrashort laser fields: An all‐electron TDDFT study

AbstractWe present a time‐dependent density functional theory (TDDFT), with proper asymptotic long‐range potential, for nonperturbative treatment of multiphoton processes of homonuclear and heteronuclear diatomic molecules in intense ultrashort laser fields. A time‐dependent two‐center generalized pseudospectral method is presented for accurate and efficient treatment of the TDDFT equations in space and time. The procedure allows nonuniform and optimal spatial grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split‐operator scheme in the energy representation is used for the time propagation of the individual molecular spin‐orbital. The theory is applied to a detailed all‐electron study of multiphoton ionization (MPI) and high‐order harmonic generation (HHG) processes of N2 and CO molecules in intense laser pulses. The results reveal intriguing and substantially different nonlinear optical response behaviors for N2 and CO, despite the fact that CO has only a very small permanent dipole moment. In particular, we found that the MPI rate for CO is higher than that of N2. Furthermore, while laser excitation of the homonuclear N2 molecule can generate only odd harmonics, both even and odd harmonics can be produced from the heteronuclear CO molecule. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Triggered multiphoton ionization

Three-photon excitation of a dipole-allowed atomic transition in rare gases is cancelled at an elevated gas pressure due to the well-known destructive interference of the three- and one-photon transition amplitudes in resonant atomic excitation. This cancellation effect is independent of laser light intensity, but is strongly modified in the presence of counterpropagating light. It has been shown that under three-photon resonant pumping at 1010 W cm−2 the multiphoton ionization through the cancelled 6s[3/2] J = 1 resonance of xenon can be initiated by counterpropagating light with an intensity as low as 103 W cm−2, which is by many orders of magnitude less than normally required for the onset of multiphoton excitation and ionization processes in rare gases. Some peculiarities of such triggered multiphoton ionization are discussed.

Multiphoton fragmentation and ionization of CF2HCl molecules and clusters by UV radiation

The results of experimental studies of multiphoton ionization of CF2HCl molecules and clusters by UV laser radiation in the wavelength range 217–236 nm are reported. In the case of molecules, the main reaction products are CF2H+ and CF+ ions as well as atomic chlorine. It is found that the spectra of the products of ionization of free molecules and molecules condensed into clusters differ qualitatively: multiphoton ionization of clusters does not yield CF2H+ ions. The dependences of the ion yield on the intensity of laser radiation and its wavelength are measured. The effect of a constant electric field and the radiation spectral width on the multiphoton ionization process is demonstrated. The shape of the velocity distributions is determined for a number of products. A strong anisotropy is detected in the reaction of formation of CF2H+ ions. Possible mechanisms for these processes are discussed.

Non-perturbative approach to multiphoton ionisation of the hydrogen and helium atoms

Laser light intensities, currently obtained in the laboratories, make it possible to observe many multi-photon phenomena in the interaction of light and matter (such as multi-photon ionisation or MPI process). Theoretical description of such phenomena necessitates the use of essentially non-perturbative framework. We present such an approach based on the recasting of the Schrodinger equation for the system atom+field into a set of coupled integral equations of the Lippmann–Schwinger type.

Laser spectroscopy on molecular beam with a time-of-flight mass spectrometer operating in a strong magnetic field

Experimental set-up for studying effects of a strong magnetic field on a structure and a decay dynamics of molecules, was designed and constructed. A vacuum chamber, in which a molecular beam propagated, was mounted in a bore of a superconducting magnet. Laser light crossed the molecular beam in the magnetic field and excited the molecules. Fragment or parent ions produced through sequential decay processes, were extracted by an electric field parallel to the magnetic field and detected by a microchannel plate. By measuring the time-of-flight from the photo-excitation to the ion-detection, a species of ions —mass and charge state— was identified. A performance of the set-up was demonstrated using the resonance enhanced multiphoton ionization process through the X2Π-A2Σ+ transition of nitric oxide (NO) molecules. A mass resolution m/Δm ≥180±6 was obtained in the field up to 10 T. This was the first successful result demonstrating the sufficient mass resolution obtained by the time-of-flight technique in the strong magnetic field up to 10 T. Parent NO+ ions were selectively detected by the mass spectrometer and the ion current was measured as a function of the frequency of the laser light. Rotational transition lines were measured with a sufficient S/N ratio in the field up to 10 T.

An experimental investigation on the resonance enhanced multiphoton ionization and dissociation processes of acetone

Resonance enhanced multiphoton ionization(REMPI) dissociation channels of acetone were studied using REMPI-TOF-MS and mass-selected excitation spectra in the range of 280—286.5nm. The mass-selected excitation spectra of fragment ions were analyzed and identified. It was shown that the main dissociation channel of acetone is the photoionization of parent molecule firstly in the range of 280—286.5nm, then followed by photodissociation of the parent ion.

Selective OD bond dissociation of HOD: Photodissociation of vibrationally excited HOD in the 5νOD state

Exclusively selective OD bond dissociation of HOD has been demonstrated by the ultraviolet photodissociation at 243.1 nm through the fourth overtone state of the OD stretching mode (5nu(OD)). Branching ratio between the OH and OD bond dissociation channels has been determined by detecting H and D atoms, utilizing a (2+1) resonance-enhanced multiphoton ionization (REMPI) process. The OD bond dissociation has been solely observed with the branching ratio phi(D+OH)/phi(H+OD)>12, which has been determined by the detection limit for the H atom. Time-dependent wave-packet calculations reveal two important features for the highly selective OD bond dissociation: (1) strong local-mode character of the 5nu(OD) state and (2) limitation of the total excitation energy lower than the saddle point between the OH and OD dissociation channels in the A state. Additionally, the recoil velocity and angular distribution of the nascent D atom are roughly evaluated by analyzing the Doppler-resolved REMPI spectrum. Based on these results, the dynamics of the selective OD dissociation has been discussed in detail.

Lippmann-Schwinger description of multiphoton ionization

We outline a formalism and develop a computational procedure to treat the process of multiphoton ionization (MPI) of atomic targets in strong laser fields. We treat the MPI process nonperturbatively as a decay phenomenon by solving a coupled set of the integral Lippmann-Schwinger equations. As basic building blocks of the theory we use a complete set of field-free atomic states, discrete and continuous. This approach should enable us to provide both the total and differential cross-sections of MPI of atoms with one or two electrons. As an illustration, we apply the proposed procedure to a simple model of MPI from a square well potential and to the hydrogen atom.

Identification, Structure, and Spectroscopy of Neutral Vanadium Oxide Clusters

Neutral vanadium oxide clusters are studied by photoionization time-of-flight (TOF) mass spectroscopy, electronic spectroscopy, and density functional theory (DFT) calculations. Mass spectra of vanadium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Mechanisms of 118 nm single photon ionization and 193 and 355 nm multiphoton ionization/fragmentation of vanadium oxide clusters are discussed on the basis of observed mass spectral patterns and line widths of the mass spectral features. Only the 118 nm laser light can ionize vanadium oxide neutral species by single photon ionization without fragmentation. The stable vanadium oxide neutral clusters under saturated oxygen growth conditions are found to be of the form (VO2)x(V2O5)y. Structures of the first few members of this series of clusters are determined through high level DFT calculations. Fragmentation of this series of clusters through 355 and 193 nm multiphoton ionization processes is discussed in light of these calculated structures. The B(2)B2 <-- X(2)A1 transition is observed for the VO2 neutral species, and nu1 and nu2 vibrations are assigned for both electronic states. From this spectrum, the VO2 rotational and vibrational temperatures are found to be approximately 50 and approximately 700 K, respectively.

Control of Local Ionization and Charge Transfer in the Bifunctional Molecule 2-Phenylethyl-N,N-dimethylamine Using Rydberg Fingerprint Spectroscopy

Local photoionization pathways and charge-transfer dynamics of 2-phenylethyl-N,N-dimethylamine (PENNA) are explored using the recently developed Rydberg fingerprint spectroscopy. PENNA, a molecule that derives its biological significance from its relation to neurotransmitters, has two ionization centers that are separated by an ethyl group. We ionize the molecule in various multiphoton ionization processes using different laser wavelengths. The Rydberg fingerprint spectrum reveals the local nature of the ionization process and identifies the center of charge. We discovered that the laser wavelength provides substantial control over the activation of the individual ionization centers. The resonant (2+1) ionization with 400-nm radiation is dominated by the ejection of an electron from the amine moiety. In contrast, the resonant (1+1) ionization with 266-nm radiation leads predominantly to an ion with the charge in the phenyl group. The clean separation of the two ionization processes allows the exploration of ultrafast charge-transfer dynamics ensuing from a specific starting state characterized by a charged phenyl moiety. The width of the corresponding spectral features suggests that the charge transfer proceeds on a femtosecond time scale, suggesting a strong coupling between the two lowest-energy electronic surfaces of the PENNA cation.

Multiphoton ionization ofHeby using intense high-order harmonics in the soft-x-ray region

We report on the multiphoton ionization processes in the soft-x-ray region $(\ensuremath{\lambda}\ensuremath{\leqslant}30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. On the basis of the measured time-of-flight spectra for both ions and electrons obtained using intense soft-x-ray pulses produced by high-order harmonics, the cross sections of the two-photon double ionization and above-threshold ionization of $\mathrm{He}$ are estimated. The high-intensity soft-x-ray radiation achieved by phase-matched high-order harmonics enables the investigation of these nonlinear optical processes, which were beyond the reach of conventional light sources.

Interaction Mechanism of Some Alkyl Iodides with Femtosecond Laser Pulses

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 x 10(13-)5 x 10(15) W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values gamma approximately 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail.

Multiphoton ionization and oxidation processes of Mg–ammonia clusters

Multiphoton ionization and oxidation processes of ammoniated magnesium clusters are investigated by the multiphoton ionization method with an intense femtosecond laser. In the photoionization of mass-selected Mg+ (NH3)n, evaporation dominates at lower laser intensity, while the oxidation reaction to produce H-atom elimination products, MgNH2+ (NH3)m, becomes predominant at higher intensity. In addition to these fragment ions, doubly-charged ions are observed for n > or = 2 at the laser intensity higher than 10(12) W cm(-2). We also examined the femtosecond pump probe experiments for Mg+ (NH3)4 by monitoring these reaction products. The lifetime of the first excited state is determined as 0.8 ps from the temporal profile of MgNH2+ (NH3)m. On the other hand, the time profile of the evaporation products exhibits a bleaching of the absorption, which gives the recovery time of the initial state as 1.2 ps. Multiphoton excitation of Mg(NH3)n with the femtosecond laser at 800 nm gives doubly-charged ions with n > 3 in addition to singly-charged cluster ions and H-atom elimination products such as MgNH2+ (NH3)m. The absence of small doubly-charged ions is ascribed to a charge reduction reaction followed by Coulomb explosion. On the basis of these results, the dynamics of the solvation and oxidation reaction processes of Mg(NH3)n is discussed.

On the Titanium Oxide Neutral Cluster Distribution in the Gas Phase: Detection through 118 nm Single-Photon and 193 nm Multiphoton Ionization

Titanium oxide clusters are generated in a supersonic expansion by laser ablation of the metal and reaction with oxygen (0.1-6%) in He expansion gas. Mass spectra of the titanium oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Only the 118 nm (10.5 eV) light can ionize Ti(m)O(n) neutral clusters without fragmentation. Both the 193 nm (6.4 eV) and 355 nm (3.5 eV) multiphoton ionization cause fragmentation of the neutral clusters during the ionization process and, thus, can complicate the determination of the stable neutral Ti(m)O(n) gas-phase species. Employing 118 nm single-photon ionization and line-width data, the Ti(m)O(2m) and Ti(m)O(2m+1) series are found to be the most stable neutral cluster species for high oxygen content in the expansion gas. Fragmentation during the multiphoton ionization process for 193 nm light yields the cluster ions Ti(m)O(2m-1,-2)+. These ions are formed by the loss of one or two oxygen atoms from Ti(m)O(2m,2m+1) neutral species. The dominant cluster growth process is suggested to be through the addition of TiO2 species. For low oxygen content (<2%) in the expansion gas, oxygen-deficient clusters of the form Ti(m)O(2m-1,-2) are also observed. These latter series are not fragmented by the 193 nm ionization process.