Enhanced Ionization Research Articles

Molecular ionization of chloromethane in strong fields: nearest neighbor gateway to highly charged ions

The strong and ultrastrong field-molecule interaction is a complex, many-body process involving multiple ionization processes. We present ion yields and molecular fragment energies for the ionization of chloromethane (CH3Cl) in a laser field with intensities spanning from 1014 to 1017 W cm−2. As the laser intensity increases, ionization of CH3Cl is observed to pass from molecular tunneling, to enhanced ionization (EI), to an atomic-like response. The energy spectra of the ions show no dependence on the intensity and has its source in dissociative molecular ionization. A classical model of an aligned C–Cl ion is used to model the interaction. Following an initial molecular ionization process, our results show EI is a driving influence in the formation of low charge states until ionization become atomic-like and involves tightly bound ion states whose ionization is unaffected by nearest neighbor ions of similar ion charge.

Polarization and molecular-orbital dependence of strong-field enhanced ionization

In this work we perform a polarization dependence study of enhanced ionization (EI) in diatomic molecules. We find that EI exists when the field polarization is parallel to the molecular axis but disappears when polarization is perpendicular. We further study EI with circular polarization and find that EI exists with circular polarization indicating that rescattering does not play a significant role for EI. Furthermore, we study molecular orbital effect on EI. We find that EI exists in \ensuremath{\sigma} type but not \ensuremath{\pi} type outmost molecular orbitals.

Electron-correlation effects in enhanced ionization of molecules: A time-dependent generalized-active-space configuration-interaction study

We numerically study models of ${\mathrm{H}}_{2}$ and $\mathrm{LiH}$ molecules, aligned collinearly with the linear polarization of the external field, to elucidate the possible role of correlation in the enhanced-ionization (EI) phenomena. Correlation is considered at different levels of approximation with the time-dependent generalized-active-space configuration-interaction method. The results of our studies show that enhanced ionization occurs in multielectron molecules and that correlation is important, and they also demonstrate significant deviations between the results of the single-active-electron approximation and more accurate configuration-interaction methods. We further investigate the role of low-lying excited states in the EI phenomena. With the inclusion of correlation we show strong carrier-envelope-phase effects in the enhanced ionization of the asymmetric heteronuclear $\mathrm{LiH}$-like molecule. The correlated calculation shows an intriguing feature of crossover in enhanced ionization with two carrier-envelope phases at critical internuclear separation.

Direct detection of enhanced ionization in CO andN2in strong fields

Enhanced ionization (EI) of molecules has been extensively studied over the past two decades as a common process in molecular dissociative ionization in strong laser fields. Direct evidence for EI has been found only in ${\mathrm{I}}_{2}$ and ${\mathrm{H}}_{2}$. However, in this work we perform a direct study of EI in CO and ${\mathrm{N}}_{2}$, and find enhanced ionization in an alternate dissociation channel in each of these two molecules following double ionization. Surprisingly, EI does not happen in the commonly seen dissociation channels that were previously assigned undergoing EI. Instead, EI occurs only in the alternate channels seen here with a lower kinetic-energy release.

Enhanced ionization of the non-symmetric HeH+ molecule driven by intense ultrashort laser pulses

We study enhanced single and double ionizations and enhanced single and double excitations in the nonsymmetric two-electron diatomic molecular ion HeH(+) in an intense ultrashort laser pulse linearly polarized along the internuclear axis (z axis). We solve a three-dimensional time-dependent Schrödinger equation, TDSE, via correlated two-electron ab initio calculations within the fixed-nuclei approximation. A complex scaling method is used for calculation of both single and double ionizations. These nonperturbative processes increase with large internuclear distance R and reach a maximum at some critical distance R(c) and decrease by further increase of R. This enhanced ionization (EI) at R(c) is accompanied by enhanced single and double excitation processes. Furthermore, EI is stronger when the permanent dipole moment of the molecule and the electric field at the peak of the laser pulse are antiparallel than when they are parallel. We predict analytically the R(c) at which the enhancement of all these molecular processes happens in HeH(+) from a simple quasistatic model and investigate the effect of Carrier Envelope Phase on these nonlinear nonperturbative processes.

Phase control of multichannel molecular high-order harmonic generation by the asymmetric diatomic molecule HeH2+in two-color laser fields

Multichannel molecular high-order harmonic generation (MHOHG) from the asymmetric diatomic molecule HeH${}^{2+}$ in two-color laser fields is investigated from numerical simulation of the corresponding time-dependent Schr\"odinger equation (TDSE). It is found that the laser-induced electron transfer (LIET) plays a crucial role in MHOHG, which leads to the multichannel harmonic generation from the ground and long-lifetime excited states. LIET is sensitive to the phase differences of the two-color laser pulses, which can be used to control the enhanced excitation (EE) and enhanced ionization (EI) of the system. Both EE and EI have a strong influence on the overall intensity of the MHOHG spectrum, and there may be four orders of magnitude difference in the MHOHG intensity between the enhanced and suppressed cases. In addition, owing to the asymmetry of the two-color laser fields and the recombination of electron with the neighboring ion, multiple cutoff energies are observed. The mechanism of these effects are confirmed by classical simulations.

Nonsymmetric molecules driven by intense few-cycle laser pulses: Phase and orientation dependence of enhanced ionization

The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses linearly polarized along the internuclear axis has been investigated. It is found that enhanced ionization (EI) occurs in nonsymmetric molecules and is accompanied by enhanced excitation (EE). We show that the nonsymmetric distribution of the electron cloud between the two nuclei leads to a strong dependence of EI and EE on the carrier envelope phase of few-cycle pulses, and on the orientation of the molecule parallel or antiparallel with the peak electric field of the pulse. This effect is as strong as the pulse duration is short and disappears with increasing pulse duration. The field ionization model, and mainly the ``energy level crossing'' mechanism, are used to explain these phase effects. The newly proposed energy level crossing mechanism, which is relevant to nonsymmetric molecules, occurs as the driving field moves the dressed ground and excited states closer to each other until their energy levels cross, leading to an enhancement of excitation and ionization. A semiclassical nonadiabatic model derived to interpret the level crossing mechanism also predicts the critical internuclear distance ${R}_{c}$ at which EI, EE, and energy crossings occur as a function of charge asymmetry and laser intensity, in good agreement with quantum-mechanical simulations.

Phase Dependence of Enhanced Ionization in Asymmetric Molecules

We study enhanced ionization (EI) in asymmetric molecules by solving the 3D time-dependent Schrödinger equation for HeH2+ driven by a few-cycle laser pulse linearly polarized along the molecular axis. We find that EI is much stronger when the laser's carrier-envelope phase is such that the electric field at the peak of the pulse is antiparallel to the permanent dipole of the molecule (PDM). This phase dependence is explained by studying the molecule in the presence of a static electric field. When this field is antiparallel to the PDM, the energy of the dressed ground state moves up (with increasing internuclear distance R) to cross with excited states, leading to a stronger ionization via intermediate state resonances and via tunneling. We predict analytically the laser and molecular parameters at which these crossings are expected to occur in any asymmetric molecule.

INFLUENCE OF RELATIVE PHASE ON THE ENHANCED IONIZATION BEHAVIOUR OF LINEAR MULTIATOMIC MOLECULAR IONS IN TWO-COLOR LASER FIELDS

The enhanced ionization(EI) behaviour of linear multiatomic molecular ions is studied in two-color(fundamental radiation:780 nm, the second harmonic:390 nm) laser fields by the numerical solution of the time-dependent Schrdinger equation with the symmetrical splitting of the short-time exponential propagator. The influence of the relative phase between the two-color laser fields on the ionization probability is given. The numerical results demonstrate that the influence of the relative phase on the ionization probability is the strongest in the range of the inter-nuclear distance where the EI occurs. The influence can be explained in terms of the field-induced over-the-barrier ionization model.

THE INFLUENCE OF THE INTENSITY AND THE FREQUENCY ON THE ENHANCED IONIZATION BEHA VIOR OF MULTIATOMIC MOLECULAR IONS IN THE INTENSE LASER FIELDS

The enhanced ionization(EI) behavior of multiatomic molecular ions is studied in intense laser fields by the numerical solution of time-dependent Schrodinger eq uation with the symmetrical splitting of the short-time exponential propagator a nd fast fourier transformation(FFT).The influence of the intensity and the frequ ency of the laser on EI is given.With the laser frequency increasing,the criticl a value of bond length for EI decreases and the ionization probability decreases too.The ionization probability increases with increasing laser intensity.The EI disappeares when the intensity reaches a certain value.

Enhanced ionization of in ultra-short intense laser fields

The ionization dependences of triatomic molecular ions exposed to ultra-short intense laser fields on the bond extension, the laser frequency, intensity and pulse envelope have been investigated numerically. We find that the ionization of is greatly enhanced when one of the two bonds stretches into a critical bond length range -6 bohr for the laser intensity . Moreover, the enhanced ionization is laser-frequency dependent, namely, the critical bond length shifts toward a small value for the short-wavelength excitation. In addition, the critical bond length is found to be variable with the laser intensity. For low-frequency excitations, the enhanced ionization (EI) effect can be explained by the field-induced over-the-barrier model. However, the EI effect is attributed to the resonance-enhanced multiphoton ionization mechanism for high-frequency (248 nm) excitation.