Role Of Rydberg States Research Articles

Cover Feature: Norbornadiene/Quadricyclane System in the Spotlight: The Role of Rydberg States and Dynamic Electronic Correlation in a Solar‐Thermal Building Block (ChemPhotoChem 4/2023)

The Cover Feature illustrates the photophysics and photochemistry of the norbornadiene (NBD) and quadricyclane (QC) molecular switch pair by the characterization of the low-lying valence and Rydberg states by means of CASPT2//CASSCF multireference theory. Two low energy conical intersections were identified for both the NBD↔QC photochemical reaction revealing that the photorelaxation is ruled by the deformation of two coupled reaction coordinates and the nonadiabatic population exchange is facilitated by a doubly excited valence state. More information can be found in the Research Article by Marco Marazzi, Mariachiara Pastore and co-workers.

Norbornadiene/Quadricyclane System in the Spotlight: The Role of Rydberg States and Dynamic Electronic Correlation in a Solar‐Thermal Building Block

AbstractIn this contribution, we studied the photophysics and photochemistry of an archetypal molecular switch in terms of solar energy storage and release. In detail, we characterized the valence and Rydberg states of norbornadiene (NBD) and quadricyclane (QC) by means of CASPT2//CASSCF theory level, finding a good agreement of the calculated vertical excitation energies with the experimental counterparts. The NBD↔QC thermal and photochemical valence isomerization [2π+2π] reactions have been addressed and investigated. Low energy crossing points between excited states were identified for both the NBD↔QC photochemical reactions, through the calculation of minimum energy paths which revealed that the photochemistry is ruled by the deformation of two coupled reaction coordinates. Such coordinates were also used to build potential energy surfaces through relaxed energy scans with the goal of building a simplified, still accurate, model system able to catch the NBD↔QC photorelaxation. Also, the S1/S0 respective conical intersection were characterized and related to the reaction quantum yield. Interestingly, we found that reverse photoisomerization from QC to the NBD progenitor could in principle occur by direct excitation of σ‐3s and σ‐3p Rydberg excited states in the UV spectral range. In both NBD and QC photoreactivity, the doubly excited valence state has shown to play a crucial role for reaching crossing points leading to nonadiabatic population transfers.

Time-resolved high harmonic spectroscopy of dynamical symmetry breaking in bi-circular laser fields: the role of Rydberg states.

The bi-circular scheme for high harmonic generation, which combines two counter-rotating circular fields with frequency ratio 2:1, has recently permitted to generate high harmonics with essentially circular polarization, opening the way for ultrafast chiral studies. This scheme produces harmonic lines at 3N + 1 and 3N + 2 multiples of the fundamental driving frequency, while the 3N lines are forbidden owing to the three-fold symmetry of the field. It is generally established that the routinely observed signals at these forbidden harmonic lines come from a slight ellipticity in the driving fields, which breaks the three-fold symmetry. We find that this is neither the only nor it is the dominant mechanism responsible. The forbidden lines can be observed even for perfectly circular, long driving pulses. We show that they encode rich information on the sub-cycle electronic dynamics that occur during the generation process. By varying the time delay and relative intensity between the two drivers, we demonstrate that when the second harmonic either precedes or is more intense than the fundamental field, the weak effects of dynamical symmetry breaking caused by finite pulse duration are amplified by electrons trapped in Rydberg orbits (i.e., Freeman resonances), and that the forbidden harmonic lines are a witness of this.

Role of Rydberg states and microwave radiation in tropospheric water-vapor cluster formation

We discuss the potential effect of incident microwave radiation from the ionosphere on the formation of condensation-cluster haze in the troposphere through modification of the cluster-ion dissociative recombination rate (primarily from water vapor).

The role of Rydberg states in photoionization of NO2 and (NO+, O−) ion pair formation induced by one VUV photon

We report a combined experimental and theoretical study of photoionization (PI) of the NO2 molecule into the NO2(+) (X (1)Σg(+)) ground state and the photodissociation of NO2 into the NO(+)((1)Σ(+)) + O(-)((2)P) ion pair. These processes were induced by 10.9 eV-13 eV synchrotron radiation and the products were detected using electron-ion or O(-)-NO(+) coincident momentum spectroscopy. The results demonstrate the strong influence of [R(∗)(4b2)(-1), nlα(i), v2(')] Rydberg states vibrationally resolved in the v2(') bending modes for both processes. In particular, we emphasize two regions around 11.5 eV and 12.5 eV that were studied in more detail for their relevance to 400 nm multiphoton ionization induced by femtosecond pulses. The photoelectron energy spectra and asymmetry parameters support the existence of two PI mechanisms, as probed with the help of fixed-nuclei frozen-core Hartree-Fock calculations. We found significant deviations from Franck-Condon ionization predictions which may be assigned to vibronic coupling of NO2(∗) states such as that induced by a conical intersection. The limited agreement between theory and experiment, even for the non-resonant processes, indicates the need for calculations that go beyond the approximations used in the current study. Ion pair formation leads to strong vibrational and rotational excitation of the NO(+)((1)Σ(+),v) product, with an ion fragment angular anisotropy depending on both the v2(') bending quantum number of the excited parent molecule and the v vibrational level of the fragment.

Role of Rydberg States in the Photostability of Heterocyclic Dimers: The Case of Pyrazole Dimer

A new route for the nonradiative decay of photoexcited, H-bonded, nitrogen-containing, heterocyclic dimers is offered and exemplified by a study of the pyrazole dimer. In some of these systems the N(3s) Rydberg state is the lowest excited singlet state. This state is formed by direct light absorption or by nonradiative transition from the allowed ππ* state. An isomer of this Rydberg state is formed by H atom transfer to the other component of the dimer. The newly formed H-bonded radical pair is composed of two radicals (a H-adduct of pyrazole, a heterocyclic analogue of the NH(4) radical) and the pyrazolium π-radical. It is calculated to have a shallow local minimum and is the lowest point on the PES of the H-pyrazole/pyrazolium radical pair. This species can cross back to the ground state of the original dimer through a relatively small energy gap and compete with the H-atom loss channel, known for the monomer. In both Rydberg dimers, an electron occupies a Rydberg orbital centered mostly on one of the two components of the dimer. This Rydberg Center Shift (RCS) mechanism, proposed earlier (Zilberg, S.; Kahan, A.; Haas, Y. Phys. Chem. Chem. Phys. 2012, 14, 8836), leads to deactivation of the electronically excited dimer while keeping it intact. It, thus, may explain the high photostability of the pyrazole dimer as well as other heterocyclic dimers.

Role of Rydberg States in the Photochemical Dynamics of Ethylene

We use the ab initio multiple spawning method with potential energy surfaces and nonadiabatic coupling vectors computed from multistate multireference perturbation theory (MSPT2) to follow the dynamics of ethylene after photoexcitation. We introduce an analytic formulation for the nonadiabatic coupling vector in the context of MSPT2 calculations. We explicitly include the low-lying 3s Rydberg state which has been neglected in previous ab initio molecular dynamics studies of this process. We find that although the 3s Rydberg state lies below the optically bright ππ* state, little population gets trapped on this state. Instead, the 3s Rydberg state is largely a spectator in the photodynamics, with little effect on the quenching mechanism or excited state lifetime. We predict the time-resolved photoelectron spectrum for ethylene and point out the signature of Rydberg state involvement that should be easily observed.

Multiphoton Ionization and Fragmentation of Hydrogen Chloride: A Diatomic Still Good for a Surprise

The competition between multiphoton ionization and fragmentation in the diatomic molecule hydrogen chloride is reviewed. Emphasis is laid on recent experimental results employing chemical imaging methods in order to obtain kinetic energy distributions and angular distributions of photoproducts. The energy range considered is 15 to 20 eV, equivalent to the absorption of three or four photons in the ultraviolet wavelength range. The role of Rydberg states as resonantly excited intermediate states in the ionization/fragmentation processes is assessed. Mixing among Σ1+ states gives rise to peculiarly shaped double minimum potential energy curves which allow for the production of hydrogen and chlorine atomic and ionic fragments via several competing pathways, in addition to the production of molecular HCl+ ions. States with different electronic properties show a qualitatively different behaviour from Σ+ states. Accidental resonances between states of differing orbital angular momentum or multiplicity serve to override these differences and cause subtle as well as significant deviations from the unperturbed behaviour.

Open problems in the physics of volume H/sup -//D/sup -/ sources

Old and new problems in the physics of magnetic multipole sources for the production of negative H/sup -//D/sup -/ ions are presented and discussed. We emphasize particularly, in this kind of plasmas, both the vibrational and electron nonequilibrium energy distributions, the role of Rydberg states in enhancing the negative ion production, the production of vibrationally excited states by atomic recombination on surfaces through different mechanisms, and the enhancement of negative ion concentrations in pulsed discharges.

Ultrafast spectroscopy of wavelength-dependent coherent photoionization cross sections of Li2 wave packets in the E1Σg+ state: The role of Rydberg states

The significance of Rydberg states in the probing (via ionization) of Li2 wave packets has been studied through quantitative measurements of the relative coherent ionization cross sections in a two-color pump–probe femtosecond experiment. Following the preparation of a single intermediate rovibronic state with a cw laser, a femtosecond pump pulse (around 800 nm) creates a single two-state rotational wave packet by coherent excitation of the E1Σg+ (ν=9; J=27 and 29) states. The wave packet is then probed through ionization using time-delayed, wavelength tunable pulses (in the region 508–690 nm) while the total energy of the system is kept below the dissociation limit of Li2+. The background-free coherent ionization yield (for each probe wavelength) is measured as the relative oscillation amplitude of the single quantum beat time-dependent signal. The experimental results closely follow a relatively simple theoretical model, which is based on the assumption that the coherent ionization predominantly takes place via the excitation of high-n bound singly excited Rydberg states in the ionization continuum converging to the X 2Σg+ ground electronic state of Li2+. The best interpretation is that the high-n Rydberg states (above n∼25) undergo collisional ionization or autoionization and contribute to the measured coherent ionization signal, while the low-n Rydberg states undergo predissociation and do not contribute to the measured signal. An implication of the results is that the final states of the Li2 system, accessed by the above probe pulses, can be better approximated by a corresponding set of isolated discrete levels rather than by a continuum. This conclusion is important to experimental, as well as theoretical, coherent control and wave packet dynamics studies, in particular, when phase- and amplitude-shaped pump and probe pulses are employed. This study is also the first to investigate ionization of lithium dimer slightly below the E 1Σg+ shelf region with visible light.

On the possible role of Rydberg states in photobiological events

On the possible role of Rydberg states in photobiological events

The very low ionization potentials of porphyrins and the possible role of rydberg states in photosynthesis

The very low ionization potentials of porphyrins lead to the prediction that in addition to bands due to (π, π *) transitions, bands due to Rydberg transitions should exist in their visible spectra. The suggestion is made that Rydberg excited states could be important in photosynthesis.