High-Resolution Photoelectron Spectroscopy of Molecules

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Here, we discuss and compare results of recent theoretical studies of rotationally resolved photoelectron spectra with measurements for molecules such as HBr, OH, NO, N_2, CO, H_2O, H_2CO, and CH_3. These studies reveal the rich dynamics of quantum-state-specific studies of molecular photoionization and provide a robust description of key spectral features resulting from Cooper minima, autoionization, alignment, partial-wave mixing, and interference in related experimental studies

Time-resolved photoelectron spectroscopy of wavepackets through a conical intersection in NO_2

We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two ^2A′ states of NO_2. The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Photoelectron angular distributions are seen to provide a clearer picture of the ionization channels and underlying wavepacket dynamics around the conical intersection than energy-resolved spectra. Time-resolved photoelectron velocity map images are also presented

Rotationally resolved photoelectron spectra in resonance enhanced multiphoton ionization of SiF

Results of calculations of rotationally resolved photoelectron spectra for resonance enhanced multiphoton ionization (REMPI) of SiF via the B ^2Σ^+ (4sσ), C" ^2Σ^+ (4pσ), and C’ ^2Π (4pπ) Rydberg states are reported. In addition to the expected ΔN=even peaks, unusually strong ΔN=±1 transitions are predicted for photoionization of the B ^2Σ^+ state. These unusual transitions are due to even angular momentum components of the photoelectron matrix element and arise from the formation of Cooper minima in the ionization channels and strong l mixing in the electronic continuum induced by the nonspherical molecular ion potential. Unexpected ΔN=0,±2 transitions, due to odd wave contributions to the photoelectron matrix element, are also predicted for photoionization of the C" ^2Σ^+ state. Asymmetrical ion distributions with respect to ΔN=0 are also predicted for the C’ ^2Π state. Cooper minima are predicted to occur in the l=2 wave of the kπ photoelectron channel for the B state and in the l=4 wave of the kσ and kπ channels for the C‘ state. Photoelectron angular distributions provide further insight into the photoionization dynamics

Rotational branching ratios and photoelectron angular distributions in resonance enhanced multiphoton ionization of diatomic molecules

In this paper we extend a previous formulation of molecular resonance enhanced multiphoton ionization (REMPI) photoelectron spectra to explicitly include multiplet‐specific final state wave functions and intermediate coupling schemes. The results of this formulation should be well suited and helpful in quantitative theoretical studies of rotationally resolved REMPI spectra in many diatomic molecules of interest. As an example, we use this formulation to study the rotational branching ratios and photoelectron angular distributions for (3+1) REMPI of NH via the 3 ^3Π Rydberg resonant state. The predicted anomalous rotational distributions are interpreted as arising from a Cooper minimum in the l=2 component of the kπ photoionization channel. A number of other results are obtained and discussed

Ion rotational distributions for nonlinear molecules at near-threshold photoelectron energies

Rotationally resolved photoelectron spectra can provide significant insight into the underlying dynamics of molecular photoionization. Recent advances in experimental techniques now make it possible to readily achieve rotational resolution in molecular photoelectron spectra. Here we discuss results of our recent theoretical studies of rotationally resolved photoelectron spectra at near-threshold energies for the nonlinear molecules H_2O, H_2S, and CH_2O (formaldehyde). These studies serve to reveal the rich dynamics of molecular photoionization and, where possible, to provide a robust description of key spectral features of interest in related experimental studies

Dynamics of multiphoton excitation and quantum diffusion in Rydberg atoms

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.39.1800.We present a detailed two-dimensional (2D) quantal study of the dynamical evolution of microwave-driven Rydberg H atoms. We examine the range of validity of the conventional one-dimensional (1D) models and explore the frequency- and intensity-dependent excitation and ionization mechanisms. The main findings of this paper can be summarized as follows: (i) The excitation spectra of Rydberg H atoms are strongly frequency dependent and can be roughly grouped into three characteristically different regions, each with a different excitation mechanism. In this paper, we emphasize the study of the two major excitation mechanisms: quantum diffusion and multiphoton resonant excitation. The region dominated by quantum diffusion lies in the frequency range ωc<ω0<ωd, where ω0 is the rescaled field frequency (ω0=ωn30; n0 is the principal quantum number of the initial state); ωc, the classical chaotic threshold; and ωd, the quantum delocalization border. In this region, quasienergy levels are strongly perturbed and mixed and excitation is efficient, leading to the so-called underthreshold photoelectric ionization phenomenon. On the other hand, we found a series of frequency regions (in ω0>ωd) where the ionization is mainly due to multiphoton resonant excitation through the more isolated quasienergy avoided crossing points. (ii) The excitation pathways (1D versus 2D) are strongly intensity dependent. For microwave (rescaled) field strength ɛ0 (≡ɛn40) in the range ɛc0 ladders rather than the n2=0 ladder, as often assumed in the 1D model. As field strength increases above ɛq, however, the 1D model improves significantly. (iii) The quantum localization phenomenon is observed in the classically chaotic region (ωcɛq. (iv) The stability of quantum diffusive motion is analyzed in terms of the quantal phase-space diagram and the autocorrelation function. The results lend support to the view that quantum mechanics can impose limitations on classical chaotic motion. (v) The way of turning on the field (sinωt or cosωt) does not affect significantly the dynamical evolution of the system. (vi) Finally, a computationally powerful new technique, invoking the use of artificial intelligence algorithms as well as the generalized Van Vleck perturbation theory for effectively reducing the dimensionality of the Floquet matrix, is introduced to facilitate the study of multiphoton resonant excitation of Rydberg atoms

Rotationally resolved near-threshold photoionization of the 1b_1 valence orbital of H_2O and D_2O

Results of theoretical studies of rotationally resolved ion distributions for near‐threshold photoionization of the 1b_1 valence orbital of H_2O and D_2O are reported and compared with measured spectra. Agreement between the calculated and measured spectra is very encouraging. The calculated and measured spectra reveal both type a and type c transitions in contrast to type c transitions only expected in an atomiclike picture. Type a transitions arise from odd (mainly p wave) angular momentum components of the photoelectron matrix elements which are due to l mixing in the electronic continua. These type a transitions are quite molecular in origin and are similar to nonatomiclike transitions seen in resonance enhanced multiphoton ionization of excited states of diatomic molecules. Useful rotational selection rules are also obtained