Nonponderomotive effects in multiphoton ionization of molecular hydrogen (original) (raw)
Observation of the Stark effect in autoionising Rydberg states of molecular hydrogen
Chemical Physics Letters, 1991
The dc Stark effect is studied for autoionising Rydberg states of H 2 converging on the ν +=2 ionisation limit. The levels ( n = 13-22) are excited from the ground state using a coherent XUV source (bandwidth ≈ 1 cm -1) and are strongly perturbed by the field (15-2000 V/cm). Many new states are observed, including the high-/hydrogenic manifolds. A detailed Stark map is obtained for the first time, and a matrix diagonalisation calculation of field-induced state mixing is carried out to explain some of the observed features.
Rydberg states in the strong field ionization of hydrogen by 800, 1200 and 1600 nm lasers
Journal of Physics B: Atomic, Molecular and Optical Physics, 2014
We study the population of Rydberg excited states in the strong field interaction of atomic hydrogen with 800, 1200 and 1600 nm laser pulses. The total excitation probability displays strong out-of-phase modulation with respect to the weak modulation in the total ionization probability as the laser intensity is increased. The results are explained in terms of channel closing, to demonstrate multiphoton ionization features in the strong tunnel ionization regime. We also explain the stability of high Rydberg states in strong laser fields in contrast to other previous ionization stabilization models.
Strong-field multiphoton ionization of hydrogen: Nondipolar asymmetry and ponderomotive scattering
Physical Review A, 1997
We present an extension of the strong-field theory of multiphoton ionization due to Reiss aimed at retaining the retardation effects. The time-reversed S-matrix approach and a recently proposed strong-field solution to the Schrödinger equation are used in order to obtain analytic expressions for the ionization rates without making the dipole approximation. Photoionization of the hydrogen ground state is used as an example to demonstrate that deviation from the dipole approximation is substantial for high photon orders or, equivalently, highly energetic photoelectrons. As a by-product, an expression is also obtained for the ponderomotive scattering angle of the photoelectron, measured relative to the field propagation direction, in terms of its directed kinetic and binding energies. ͓S1050-2947͑97͒07111-4͔
Semiclassical analysis of long-wavelength multiphoton processes: The Rydberg atom
Physical Review A, 2004
We study the problem of multiphoton processes for intense, long-wavelength irradiation of atomic and molecular electrons. An exact, nonperturbative approach is applied to the standard vector potential coupling Hamiltonian for a three-dimensional hydrogenlike atom in a microwave field treated semiclassically. Multiphoton probability exchange is calculated in both the velocity and the length gauges, by applying the Goeppert-Mayer gauge transformation. The expansion of the time-dependent solution in terms of Floquet states delineates the mechanism of multiphoton transitions. A detailed analysis of the Floquet states and quasienergies as functions of the field parameters allows us to describe the relation between avoided quasienergy crossings and multiphoton probability exchange. We formulate analytical expressions for the variation of quasienergies and Floquet states with respect to the field parameters, and demonstrate that avoided quasienergy crossings are accompanied by dramatic changes in the Floquet states. Analysis of the Floquet states, for small values of the field strength, yields selection rules for the avoided quasienergy crossings. In the case of strong fields, the simultaneous choice of frequency and strength of the field producing an avoided crossing results in improved ionization probability.
Photo-double ionization of molecular hydrogen
Journal of Physics B: Atomic, Molecular and …, 1999
The angular distribution of the correlated electron pair emitted in single-photon double ionization of the hydrogen molecule is analysed and calculated using a variety of approximations. Attention is directed particularly towards the differences between the molecular angular distribution and that of the corresponding 'united atom', i.e. that arising from the double ionization of helium. Qualitative agreement is obtained with recent experiments on photo-double ionization of the hydrogen molecule. The major effects arising from the two-centre nuclear field of the molecule and the orientation of the axis at the moment of photon absorption are exposed in the simpler problem of photoionization of the H + 2 ion.
We discuss the spectrum of very high Rydberg states as detected via ionization in weak external electric fields. For the conditions of interest, namely, states just below the ionization continuum and weak fields, the classical barrier to dissociation is extremely far out from the core. About the saddle point the potential is very shallow. It is concluded that ionization by tunneling is far too slow. Only electrons whose energy is above the classical barrier can be detected via ionization. However, not all electrons which energetically can ionize will necessarily do so. Electrons may fail to ionize if the fraction of their energy which is in the direction perpendicular to the field is high. The computed fraction of electrons which fails to ionize does depend, in a sensitive way, on the diabatic vs adiabatic switching on of the external field. More experiments and theoretical work is needed on this point. A classical procedure based on the adiabatic invariance of the volume in phase space is developed for the computation of the fraction of electrons that can surmount the classical barrier for a given field. Analytically exact results are obtained for adiabatic switching and for the sudden limit where the rise time of the field is shorter than the period of the orbit. For the case of diabatic switching (which is appropriate for very high n values), the exact classical computations on the yield of ionization show that the onset of ionization is at an energy of 4.25 F1I2 cm-I below the ionization potential and the 50% point it as 3.7 F1I2 cm-I for a field F in V/cm.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2011
The ionization of the ground state hydrogen atom due to the simultaneous interaction with a laser ¢eld and its third harmonic is analyzed in the case in which both components of the two-color ¢eld have identical elliptic polarizations. The laser frequencies are chosen in the vicinity of the two-photon resonances. In the framework of the third order perturbation theory, the in£uence of the light helicity on the di¡erential rate is investigated. Changes in the alignment of the azimuthal angular distribution of the ejected photoelectrons, induced by the harmonic relative phase, are reported.
Spectroscopic Observation and Characterization of H + H − Heavy Rydberg States †
The Journal of Physical Chemistry A, 2009
A series of discrete resonances was observed in the spectrum of H 2 , which can be unambiguously assigned to bound quantum states in the 1/R Coulombic potential of the H + Hion-pair system. Two-step laser excitation was performed, using tunable extreme ultraviolet radiation at λ) 94-96 nm in the first step, and tunable ultraviolet radiation in the range λ) 310-350 nm in the second step. The resonances, detected via H + and H 2 + ions produced in the decay process, follow a sequence of principal quantum numbers (n) 140-230) associated with a Rydberg formula in which the Rydberg constant is mass scaled. The series converges upon the ionic H + Hdissociation threshold. This limit can be calculated without further assumptions from known ionization and dissociation energies in the hydrogen system and the electronegativity of the hydrogen atom. A possible excitation mechanism is discussed in terms of a complex resonance. Detailed measurements are performed to unravel and quantify the decay of the heavy Rydberg states into molecular H 2 + ions, as well as into atomic fragments, both H(n) 2) and H(n) 3). Lifetimes are found to scale as n 3 .
Measurement of high Rydberg states and the ionization potential of H2
Physical Review A, 1989
We have measured numerous transitions from the E,F state of H2 to the singlet np Rydberg states with an accuracy of 0.01 cm '. The data have been analyzed to obtain values for the ionization potential of H2 relative to the v=0, N 0 and 1 levels of the E,F state. By combining these results with previous measurements of the E,F state, a value of 124417.524~0.015 cm ' is obtained for the ionization potential of H2. This result is in good agreement with previous work but nearly an order of magnitude more accurate, and is very close to the best theoretical value.