Resonant Multiphoton Excitation Research Articles

Resonant multiphoton processes and excitation limits to structural dynamics.

Understanding the chemical reactions that give rise to functional biological systems is at the core of structural biology. As techniques are developed to study the chemical reactions that drive biological processes, it must be ensured that the reaction occurring is indeed a biologically relevant pathway. There is mounting evidence indicating that there has been a propagation of systematic error in the study of photoactive biological processes; the optical methods used to probe the structural dynamics of light activated protein functions have failed to ensure that the photoexcitation prepares a well-defined initial state relevant to the biological process of interest. Photoexcitation in nature occurs in the linear (one-photon per chromophore) regime; however, the extreme excitation conditions used experimentally give rise to biologically irrelevant multiphoton absorption. To evaluate and ensure the biological relevance of past and future experiments, a theoretical framework has been developed to determine the excitation conditions, which lead to resonant multiphoton absorption (RMPA) and thus define the excitation limit in general for the study of structural dynamics within the 1-photon excitation regime. Here, we apply the theoretical model to bacteriorhodopsin (bR) and show that RMPA occurs when excitation conditions exceed the linear saturation threshold, well below typical excitation conditions used in this class of experiments. This work provides the guidelines to ensure excitation in the linear 1-photon regime is relevant to biological and chemical processes.

Vibration-assisted multiphoton resonance and multi-ion excitation

We investigate the multiphoton resonance and multi-ion excitation in a single-mode cavity with identical vibrating ionqubits, which enables the tripartite interaction among the internal states of ions, the cavity mode and the ions' vibrational motion. Under particular resonant conditions, we derive effective Hamiltonians for the three-photon and the three-excitation cases, respectively, and find that the magnitude of the effective coupling energy can be tuned through the vibration mode, allowing for manipulations of ion-photon coupling in experiments. Furthermore, we analyze the system dynamics of our proposed setups and demonstrate the Rabi oscillation behaviors in these systems with dissipation effects. We propose our system as a versatile platform for the exploration of entangled multiqubit physics.

Rydberg state excitation in molecules manipulated by bicircular two-color laser pulses

Multiphoton resonant excitation and frustrated tunneling ionization, manifesting the photonic and optical nature of the driving light via direct excitation and electron recapture, respectively, are complementary mechanisms to access Rydberg state excitation (RSE) of atoms and molecules in an intense laser field. However, clear identification and manipulation of their individual contributions in the light-induced RSE process remain experimentally challenging. Here, we bridge this gap by exploring the dissociative and nondissociative RSE of H2 molecules using bicircular two-color laser pulses. Depending on the relative field strength and polarization helicity of the two colors, the RSE probability can be boosted by more than one order of magnitude by exploiting the laser waveform-dependent field effect. The role of the photon effect is readily strengthened with increasing relative strength of the second-harmonic field of the two colors regardless of the polarization helicity. As compared to the nondissociative RSE forming H2 * , the field effect in producing the dissociative RSE channel of ( H + , H * ) is moderately suppressed, which is primarily accessed via a three-step sequential process separated by molecular bond stretching. Our work paves the way toward a comprehensive understanding of the interplay of the underlying field and photon effects in the strong-field RSE process, as well as facilitating the generation of Rydberg states optimized with tailored characteristics.

High-resolution photoelectron spectroscopy of the ground and first excited electronic states of MgKr+

We report on the characterisation of the lowest three electronic states of MgKr, the X ground state associated with the Mg(3s) + Kr(4p) dissociation limit and the A and B excited states associated with the Mg(3p) + Kr(4p) dissociation limits. The vibrational structure of the X ground state was measured by pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy starting from the a metastable state of MgKr generated by laser ablation of a Mg rod in a Kr supersonic beam. The vibrational assignment was derived from the isotopic structure of the spectra and the adiabatic ionisation energy of the ionising transition was determined to be 38182.4(2.0) cm. Partially rotationally resolved spectra of the A X and B X transitions were recorded by monitoring the yield of Mg ions produced by dissociation of MgKr using the technique of isolated-core Rydberg-dissociation spectroscopy. Analytical potential-energy functions of the X, A, and B states were derived using a global model of these states that includes the spin-orbit interaction. The observation of the highest vibrational levels of the B state and the onset of the Kr + Mg(3p) dissociation continuum enabled the precise determination of the dissociation energies of the X, A and B states of MgKr and of the a state of MgKr. The X, A and B states of MgKr are important stepping stones to access the Rydberg states of MgKr and the ground state of MgKr by resonant multiphoton excitation.

On the Possibility of the Effective Isotope-Selective Infrared Dissociation of 235UF6 Molecules Vibrationally Excited by Bichromatic Laser Radiation

Using the spectroscopic data on the 235UF6 and 238UF6 molecules and on the lasing frequencies of CF4 and para-H2 lasers and recent results, a method has been proposed to increase the efficiency of the isotope-selective infrared laser dissociation of 235UF6 molecules under nonequilibrium thermodynamic shock conditions. The method involves two processes: (i) the resonant multiphoton excitation of 235UF6 molecules to the 3ν3 or 2ν3 vibrational states by the bichromatic infrared radiation of two CF4 or para-H2 lasers and (ii) the irradiation of 235UF6 molecules with SF6 molecules serving as sensitizers resonantly absorbing the radiation of these lasers. The essence of the method has been described. Schemes and parameters for isotope-selective dissociation of 235UF6 molecules using this method has been presented.

All-optical Stückelberg spectroscopy of strongly driven Rydberg states

The AC Stark shift of electronic levels is ubiquitous in the interaction of intense light fields with atoms and molecules. As the light intensity changes on the rising and falling edges of a femtosecond laser pulse, it shifts the Rydberg states in and out of multiphoton resonances with the ground state. The two resonant pathways for transient excitation arising at the leading and the trailing edges of the pulse generate Young's type interference, generally referred to as the St\"uckelberg oscillations. Here we report the observation of the St\"uckelberg oscillations in the intensity of the coherent free-induction decay following resonant multiphoton excitation. Moreover, combining the experimental results with accurate numerical simulations and a simple model, we use the St\"uckelberg oscillations to recover the population dynamics of strongly driven Rydberg states inside the laser pulse by all-optical measurements after the end of the pulse. We demonstrate the potential of this spectroscopy to characterize lifetimes of Rydberg states dressed by laser fields with strengths far exceeding the Coulomb field between the Rydberg electron and the core.

Strong-field dissociative Rydberg excitation of oxygen molecules: Electron-nuclear correlation

We experimentally investigate strong-field dissociative Rydberg excitation (DRE) of ${\mathrm{O}}_{2}$ molecules by using the photoelectron photoion coincidence spectrum, in which the outgoing neutral Rydberg fragments created by the strong laser fields are postionized by the weak static dc field of the spectrometer and afterwards detected as charged particles. The peak positions of the sum-kinetic energy release (KER) spectrum of the ejected nuclear fragments and electrons of the dissociative single ionization (DSI) channel are measured to be similar to that of the DRE channel, although their nuclear KER spectra are distinct. It indicates that the same number of photons is absorbed by the molecule in accessing the DRE and DSI channels, which are partitioned by the electron and nuclear fragments with various proportions. Different pathways towards the DRE and DSI channels are discussed based on the correlated dynamics of the nuclear fragments and electrons. Our results show that the DRE channel observed here is mostly accessed by the multiphoton resonance excitation process.

Electron-nuclear correlated multiphoton-route to Rydberg fragments of molecules

Atoms and molecules exposed to strong laser fields can be excited to the Rydberg states with very high principal quantum numbers and large orbitals. It allows acceleration of neutral particles, generate near-threshold harmonics, and reveal multiphoton Rabi oscillations and rich photoelectron spectra. However, the physical mechanism of Rydberg state excitation in strong laser fields is yet a puzzle. Here, we identify the electron-nuclear correlated multiphoton excitation as the general mechanism by coincidently measuring all charged and neutral fragments ejected from a H2 molecule. Ruled by the ac-Stark effect, the internuclear separation for resonant multiphoton excitation varies with the laser intensity. It alters the photon energy partition between the ejected electrons and nuclei and thus leads to distinct kinetic energy spectra of the nuclear fragments. The electron-nuclear correlation offers an alternative visual angle to capture rich ultrafast processes of complex molecules.

Comparison Study of Strong-Field Ionization of Molecules and Atoms by Bicircular Two-Color Femtosecond Laser Pulses.

We experimentally investigate the single and double ionization of N_{2} and O_{2} molecules in bicircular two-color femtosecond laser pulses, and compare with their companion atoms of Ar and Xe with comparable ionization thresholds. Electron recollision assisted enhanced ionization is observed in N_{2} and Ar by controlling the helicity and field ratio between the two colors, whereas the enhanced ionization via the recollision is almost absent in O_{2} and Xe. Our S-matrix simulations clearly reveal the crucial role of the detailed electronic structures of N_{2} and O_{2} on the two-dimensional recollision of the electrons driven by the bicircular two-color laser fields. As compared to Ar, the resonant multiphoton excitation dominates the double ionization of Xe.

Enhanced high-order-harmonic generation and wave mixing via two-color multiphoton excitation of atoms and molecules

We consider harmonics generation and wave-mixing by two-color multi photon resonant excitation of three-level atoms/molecules in strong laser fields. The coherent part of the spectra corresponding to multicolor harmonics generation is investigated. The obtained analytical results on the basis of generalized rotating wave approximation are in a good agreement with numerical calculations. The results applied to the hydrogen atom and homonuclear diatomic molecular ion show that one can achieve efficient generation of moderately high multicolor harmonics via multiphoton resonant excitation by appropriate laser pulses.

Helicity sensitive enhancement of strong-field ionization in circularly polarized laser fields.

We investigate the strong-field ionization from p± orbitals driven by circularly polarized laser fields by solving the two-dimensional time-dependent Schrödinger equation in polar coordinates with the Lagrange mesh technique. Enhancement of ionization is found in the deep multiphoton ionization regime when the helicity of the laser field is opposite to that of the p electron, while this enhancement is suppressed when the helicities are the same. It is found that the enhancement of ionization is attributed to the multiphoton resonant excitation. The helicity sensitivity of the resonant enhancement is related to the different excitation-ionization channels in left and right circularly polarized laser fields.

Multiphoton dynamics of qutrits in the ultrastrong coupling regime with a quantized photonic field

Multiphoton resonant excitation of a three-state quantum system (a qutrit) with a single-mode photonic field is considered in the ultrastrong coupling regime, when the qutrit–photonic field coupling rate is comparable to appreciable fractions of the photon frequency. For ultrastrong couplings, the obtained solutions of the Schrodinger equation that reveal multiphoton Rabi oscillations in qutrits with the interference effects leading to the collapse and revival of atomic excitation probabilities at the direct multiphoton resonant transitions.

Generation of harmonics via multiphoton resonant excitation of hydrogenlike ions in an x-ray free-electron-laser field

The coherent radiation spectrum of highly charged hydrogenlike ions in an intense x-ray free-electron-laser field is considered. The spectrum corresponding to harmonic generation in the resonant multiphoton excitation regime is investigated both analytically and numerically, arising from the Dirac Hamiltonian. The obtained analytical results are based on the generalized rotating wave approximation and are in good agreement with performed numerical calculations. Estimations show that one can achieve efficient generation of coherent hard x-ray radiation using multiphoton resonant excitation by appropriate x-ray pulses.

Multiphoton resonant excitation of Fermi-Dirac sea in graphene at the interaction with strong laser fields

In order to develop the microscopic theory of the multiphoton interaction of graphene with strong laser fields of arbitrary polarization, the analytical solution of established evolutionary equation for graphene single-particle density matrix was obtained revealing the generalized Rabi oscillations of Fermi-Dirac sea in the graphene. Creation of coherent superposition states of the particle-hole pairs in the graphene at the multiphoton resonant excitation by laser pulses of arbitrary polarizations and harmonics generation at the particle-hole annihilation from these superposition states are shown.

Creation of particle-hole superposition states in graphene at multiphoton resonant excitation by laser radiation

Nonlinear dynamics of the establishment of electron-hole coherent superposition states in graphene by multiphoton resonant excitation of interband transitions in laser fields is considered. The single-particle time-dependent density matrix for such a quantized system is calculated in the multiphoton resonant approximation. The dependence of Rabi oscillations of the Fermi-Dirac sea in graphene on the time, momentum, and photon number at multiphoton laser excitation is analyzed.

Coherent radiation by two-level quantum systems with permanent dipole moments under multiphoton resonant laser excitation

The theory of coherent radiation emission in two-level quantum systems with permanent dipole moments at the multiphoton resonant excitation by moderately strong laser fields is presented. A simple analytic formula for the expectation value of a dipole moment operator is obtained. The spectrum corresponding to harmonic generation and low-frequency radiation on the generalized Rabi frequency is investigated both analytically and numerically. The effect of dynamic Stark shift on a dipole spectrum under direct multiphoton resonant excitation is considered.

An analytic algebraic approach to study the influence of molecular alignment and orientation on multiphoton excitation in intense laser fields

The influence of molecular alignment and orientation on multiphoton vibrational excitation of diatomic molecules H2, HD, and D2 is studied by an analytical algebraic approach. The explicit expressions of the time-evolution operator and the excitation probability are given. The long-time average absorbed energy spectra and the time-dependent absorbed energy are obtained. Results show that the impact of molecular orientation on the multiphoton resonant excitation is decided by the molecular type and molecular anharmonicity. This is valuable for controlling the vibrational excitation of molecules, which is relevant to the whole field of molecular physics and physical chemistry. Furthermore, good agreement with numerical simulation is achieved.

Tunable high-power terahertz radiation generation in three-level atomic and molecular systems

A scheme is presented for generation of powerful coherent terahertz radiation by multiphoton resonant excitation of three-level hydrogenlike atoms and homonuclear diatomic molecules or molecular ions with strong laser pulses. The results of analytical and numerical investigations of the problem show that with the proposed mechanism one can achieve the implementation of widely tunable powerful terahertz sources, which may open new perspectives for terahertz science and its applications.

Vibronic structure of the 3 s Rydberg state of the 2-methylallyl radical

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled 2-methylallyl (C 4H 7) radicals provides vibronic spectra of the 3 s and 3 p Rydberg states. Analysis of the vibronic structure following one-photon and two-photon excitation of rovibronically cold 2-methylallyl radicals and its isotopologues C 4H 4D 3 and C 4D 7 reveals transitions to more than 30 vibrational levels in the 3 s Rydberg state that are identified and reassigned on the basis of predictions from ab initio calculations and results from pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained with resonant multiphoton excitation via selected intermediate states. Depletion spectroscopy reveals transitions to short-lived 3 p Rydberg states that have a large oscillator strength.

Vibronic Structure of the 3s and 3p Rydberg States of the Allyl Radical

Resonance-enhanced multiphoton ionization combined with electronic ground state depletion spectroscopy of jet-cooled allyl radicals (C(3)H(5)) provides vibronic spectra of the 3s and 3p Rydberg states. Analysis of the vibronic structure following two-photon excitation of rovibrationally cold allyl radicals reveals transitions to the 3p(z) ((2)A(1)) Rydberg state with an electronic origin at 42230 cm(-1). More than 40 transitions to vibrational levels in the partially overlapping spectra of the 3p(y) ((2)B(2)) Rydberg state and the 3s ((2)A(1)) Rydberg state are identified and reassigned on the basis of predictions from ab initio calculations and results and simulations of pulsed-field-ionization zero-kinetic-energy photoelectron spectra obtained recently using resonant multiphoton excitation via selected vibrational levels of these two Rydberg states (J. Chem. Phys. 2009, 131, 014304). Depletion spectroscopy reveals that the transition to the short-lived 3p(x) ((2)B(1)) Rydberg state in vicinity of three-state same symmetry conical intersections predicted theoretically carries most of the oscillator strength of these coupled 3s and 3p Rydberg states. The results allow for the first time to experimentally derive the energetic ordering of the 3p Rydberg states of the allyl radical.