Henry Fielding

Electric field effects on zero-kinetic-energy photoelectron spectra: An explanation of observed trends

Abstract An explanation is offered for the propensity for large negative changes of rotational angular momentum in ZEKE photoelectron spectra, which is increasingly observed as the rotational constant of the photoion decreases; Stark mixing becomes progressively more important, facilitating multistep rotational autoionization processes. The Inglis—Teller limit serves as a useful benchmark for assessing the importance of electric field effects. A discussion is presented of the relative importance of l mixing and J mixing.

Photoionisation and ZEKE photoelectron spectroscopy of Ar, H2 and CO2 using a coherent XUV laser source

Abstract Tunable extreme ultraviolet (XUV) radiation of narrow bandwidth (0.6 cm −1 ) is generated in the region of 130000 cm −1 . This radiation is successfully applied to the study of autoionising Rydberg states of argon and hydrogen, and to record one-photon zero kinetic energy (ZEKE) photoelectron spectra of argon and carbon dioxide.

Taking the green fluorescence out of the protein: dynamics of the isolated GFP chromophore anion

The green fluorescent protein (GFP) is employed extensively as a marker in biology and the life sciences as a result of its spectacular fluorescence properties. Here, we employ femtosecond time-resolved photoelectron spectroscopy to investigate the ultrafast excited state dynamics of the isolated GFP chromophore anion. Excited state population is found to decay bi-exponentially, with characteristic lifetimes of 330 fs and 1.4 ps. Distinct photoelectron spectra can be assigned to each of these timescales and point to the presence of a transient intermediate along the decay coordinate. Guided by ab initio calculations, we assign these observations to twisting about the C–C–C bridge followed by internal conversion to the anion ground state. The dynamics in vacuo are very similar to those observed in solution, despite the difference in absorption spectra between the two media. This is consistent with the protein environment restricting rotation about the C–C–C bond in order to prevent ultrafast internal conversion and preserve the fluorescence.

Photodetachment Spectra of Deprotonated Fluorescent Protein Chromophore Anions

Isolated model anion chromophores of the green and cyan fluorescent proteins were generated in an electrospray ion source, and their photodetachment spectra were recorded using photoelectron imaging. Vertical photodetachment energies of 2.85(10) and 4.08(10) eV have been measured for the model green fluorescent protein chromophore anion, corresponding to photodetachment from the ground electronic state of the anion to the ground and first excited electronic states of the radical, respectively. For the model cyan fluorescent protein chromophore anion, vertical photodetachment energies of 2.88(10) and 3.96(10) eV have been measured, corresponding to detachment from the ground electronic state of the anion to the ground and first excited electronic states of the neutral radical, respectively. We also find evidence suggesting that autoionization of electronically excited states of the chromophore anions competes with direct photodetachment. For comparison and to benchmark our measurements, the vertical photodetachment energies of deprotonated phenol and indole anions have also been recorded and presented. Quantum chemistry calculations support our assignments. We discuss our results in the context of the isolated protein chromophore anions acting as electron donors, one of their potential biological functions.

Ultrafast dynamics of aniline following 269–238 nm excitation and the role of the S2(π3s/πσ*) state

Femtosecond time-resolved photoelectron imaging is employed to investigate ultrafast electronic relaxation in aniline, a prototypical aromatic amine. The molecule is excited at wavelengths between 269 and 238 nm. We observe that the S2(π3s/πσ*) state is populated directly during the excitation process at all wavelengths and that the population bifurcates to two decay pathways. One of these involves ultrafast relaxation from the Rydberg component of S2(π3s/πσ*) to the S1(ππ)* state, from which it relaxes back to the electronic ground state on a much longer timescale. The other appears to involve motion along the πσ* dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S3(ππ*) state, which undergoes extremely efficient electronic relaxation back to the ground state. Our study supports some conclusions reached from H-atom photofragment translational spectroscopy measurements and pump–probe photoionization measurements and contradicts some others.

Observation and control of dissociating and autoionizing Rydberg electron wave packets in NO

The dynamics of predissociating Rydberg electron wave packets are observed using the optical Ramsey method. The time-resolved spectra are hydrogenic and are very well modeled by assuming that only one p Rydberg series contributes to the dynamics. This is in contrast with previous observations of autoionizing Rydberg electron wave packets [Phys. Rev. Lett. 83, 2552 (1999)], which show quite dramatic deviations from hydrogenic behavior above the Born–Oppenheimer limit. The origin of these deviations lies in the interplay between electronic and molecular phase. By exploiting these phases we are able to control the ratio of predissociaton to autoionization.

Excitation, dynamics, and control of rotationally autoionizing Rydberg states of H2

The dynamics of rotationally autoionizing Rydberg states of molecular hydrogen is investigated using a time-dependent extension of multichannel quantum defect theory, in which the time-dependent wave packets are constructed using first-order perturbation theory. An analytical expression for the complex excitation function for a sequence of Gaussian excitation pulses is derived and then employed to investigate the influence of pairs of pulses with well-defined phase differences on the decay dynamics and final-state composition.

Interfering Rydberg wave packets in Na

This paper presents an experimental and theoretical analysis of quantum interference between Rydberg wave packets in Na. Pairs of phase-locked wave packets manipulate the total orbital angular momentum of Na Rydberg atoms. Initially, the wave packet is composed of a superposition of s and d Rydberg series. Exploitation of the difference between the quantum defects of the two series allows one to predict the phase of the second wave packet required to engineer specific angular momentum compositions within the resultant wave packet. Experimentally, this final quantum state distribution is analysed in the frequency domain using state-selective field ionization and in the time domain using the optical Ramsey method. The theoretical calculations show how the phase difference between pairs of optical pulses is linked to the corresponding Rydberg frequency spectrum, therefore enabling the control of the quantum state composition of the wave packets. Finally, it is shown that by intuitively chirping one of the laser pulses it is possible to compensate for the dispersion of the wave packet and improve the effectiveness of the angular momentum control.

Observation of the Stark effect in autoionising Rydberg states of molecular hydrogen

Abstract 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.

Development of a new photoelectron spectroscopy instrument combining an electrospray ion source and photoelectron imaging

A new apparatus has been constructed that combines electrospray ionization with a quadrupole mass filter, hexapole ion trap, and velocity-map imaging. The purpose is to record photoelectron images of isolated chromophore anions. To demonstrate the capability of our instrument we have recorded the photodetachment spectra of isolated deprotonated phenol and indole anions. To our knowledge, this is the first time that the photodetachment energy of the deprotonated indole anion has been recorded.