ALBERT

Notes: Doubly enhanced sum-frequency mixing (DESMI) of two collinear laser beams is studied using a pulsed jet of carbon monoxide as the nonlinear medium. The pump laser frequency (ω1) is in two-photon resonance with a selected rotational component of the A1Π–X1Σ+ (5,0) transition. Tuning the probe laser frequency (ω2) to a sequential rotational component of the B1Σ+–A1Π (1,5) transition leads to strong enhancement of VUV generation at frequency 2ω1+ω2. The double enhancement is shown to be subject to firm overall selection rules. It is demonstrated that DESMI can be accurate and efficient for the study of molecular states in the VUV and XUV provided that low laser powers are used. The high selectivity of the four-wave mixing processes investigated allows a better understanding of the competition between four-wave mixing, reabsorption of the sum frequency, and multiphoton absorption. In particular, it is shown that resonance-enhanced multiphoton absorption is an important saturation mechanism at high laser powers.

Notes: Sequences of pulsed electric fields have been designed and tested that enable a higher selectivity in the pulsed field ionization of high Rydberg states (n≥100) than has so far been possible. The enhanced selectivity originates from the permutation of the parabolic quantum numbers n1 and n2 that is induced by a sufficiently rapid inversion of the electric field polarity during a pulse sequence. A reliable procedure, based on numerical simulations of the outcome of pulse field ionization sequences, has been developed to detect and control changes in the parabolic quantum numbers that can occur during a pulse sequence. The procedure can be used to assess under which conditions a clean permutation of the parabolic quantum numbers can be achieved. Unwanted randomization of m, n1 and n2, which reduces the selectivity of the field ionization process, can be avoided by minimizing the time intervals during which the electric field in the pulse sequence is almost zero. The high selectivity reached in the pulsed field ionization of high Rydberg states has been used to record pulsed-field-ionization zero-kinetic-energy photoelectron spectra of argon and nitrogen at an unprecedented resolution of 0.06 cm−1. This resolution opens new perspectives in photoelectron spectroscopy. © 2001 American Institute of Physics.

Notes: The pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectrum of Kr2 has been recorded between 103 500 cm−1 and 118 000 cm−1. Photoelectronic transitions to four [the I(1/2u), I(3/2u), II(1/2u), and II(1/2g) states] of the first six electronic states of Kr2+ have been observed. The photoelectronic transition to the ground I(1/2u) state consists of a long progression of vibrational bands, starting at v+=0. From the resolved isotopic substructure of vibrational levels with v+≥15, the absolute numbering of the vibrational quantum number could be determined. The analysis of the spectrum has led to improved values of the adiabatic ionization potential [IP(I(1/2u))=(103 773.6±2.0) cm−1], the dissociation energy [D0+(I(1/2u))=(9267.8±2.8) cm−1] and to the determination of an analytical potential energy curve that reproduces the experimental data from v+=0 to beyond 81% of the dissociation energy. The transitions to vibrational levels of the I(1/2u) state with v+≤30 and v+≥65 have vanishing Franck–Condon factors for direct ionization from the ground neutral state and gain intensity from transitions to low Rydberg states that belong to series converging on excited electronic states of Kr2+. In the region immediately below the first dissociation limit of Kr2+, a second progression was observed and assigned to a photoelectronic transition to the I(3/2u) state. The adiabatic ionization potential [IP(I(3/2u))=(112 672.4±2.0) cm−1], the dissociation energy [D0+(I(3/2u))=(369.1±2.8) cm−1] and vibrational constants could be extracted for this state. Two further progressions were observed below the second dissociation limit of Kr2+ and assigned to transitions to the II(1/2u) and II(1/2g) states. The adiabatic ionization potentials [IP(II(1/2u))=(117 339.7±2.0) cm−1, IP(II(1/2g))=(117 802.6±2.0) cm−1] and the dissociation energies [D0+(II(1/2u))=(1071.7±2.8) cm−1, D0+(II(1/2g))=(608.8±2.8) cm−1] were determined for these two ionic states. In the region just below the ionic dissociation limits, artifact lines are observed in the PFI-ZEKE photoelectron spectra at the position of transitions to Rydberg states of the krypton monomer. At the lowest threshold, collisional and associative ionization of the long lived atomic Rydberg states leads to the formation of ZEKE electrons; at the upper threshold, the rapid autoionization of the atomic Rydberg states forms high ion concentrations, and the electrons that remain trapped in the ion cloud are released by the delayed pulsed field used to produce and extract the PFI-ZEKE electrons. © 2001 American Institute of Physics.

Notes: Millimeter wave spectroscopy has been used to record high-resolution spectra of high-n (n=51–64), low-l (l=1–3) Rydberg states of ortho H2 located below the N+=1 rotational level of the X 2Σg+(v+=0) ground vibronic state of H2+. The spectral resolution of better than 1 MHz enables the observation of the hyperfine structure in these spectra. A simple procedure, based on the determination of combination differences, is used to reconstruct the energy level structure in np, nd, and nf Rydberg states of H2. The Stark effect is used to distinguish experimentally between p and f Rydberg states. In the weakly penetrating nf series, the hyperfine interaction dominates and the observed hyperfine components are of mixed singlet (S=0) and triplet (S=1) character. In the penetrating np series, the dominant interactions are between the electron orbital and spin angular momenta and the molecular rotation and the observed hyperfine components are characterized by a well-defined total electron spin. The nd Rydberg states show a behavior intermediate between these two limiting cases. The observed levels are of mixed singlet (S=0) and triplet (S=1) character but the main energy separation departs from the energy separation between the Gc=1/2 and Gc=3/2 levels of the H2+ ion. © 2000 American Institute of Physics.

Notes: The adiabatic ionization potential of methylene has been determined to be 83772±3 cm−1 from a rotationally resolved photoelectron spectroscopic study of the CH2+ X˜ 2A1 (0,0,0)←CH2 X˜ 3B1(0,0,0) transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and CH2+ D0(CH2(Double Bond)CO+)=33202±7 cm−1, the 0 K dissociation energy of the methyl radical D0(CH2–H)=38179±49 cm−1, the 0 K dissociation threshold of methane in CH2 and H2 D0(CH2–H2)=38232±50 cm−1 and the 0 K enthalpy of formation of CH2 ΔfH(minus sign in circle)(CH2,T=0 K)=390.73±0.66 kJ mol−1. © 2002 American Institute of Physics.

Notes: The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+=0,1,2,3) of the ground vibronic X 2Σ+g (v+=0) state of the N+2 cation have been investigated by delayed pulsed field ionization (PFI) following two-photon enhanced (2+1′) three-photon excitation via the a″ 1Σ+g (v′=0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200–2000 ions/mm3. All Rydberg states in the range of principal quantum number n=140–200 exhibit extreme stability against autoionization and predissociation and some have lifetimes which exceed 30 μs. The decay of the highest Rydberg states beyond n=200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n=350 can be observed by delayed PFI. The Rydberg states which converge on the N+=0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into neutral products (by a process presumed to be predissociation) in less than 7 μs in the range n〈100. The importance of predissociation is greatly reduced beyond n=100 and becomes negligible on our experimental timescale (30 μs) above n=140. The decay of the Rydberg states converging on the N+=2 and 3 rotational levels of the ion is more complex. Below n=100, only 30%–40% of the Rydberg population decays by fast rotational autoionization whereas 60%–70% decays by predissociation.The importance of predissociation decreases rapidly above n=100 and becomes negligible beyond n=140. The decay by rotational autoionization can be observed at all n values but becomes noticeably slower beyond n=100. In the range n=140–200 it exhibits a marked biexponential decaying behavior with 30% of the population decaying within a few microseconds and 70% displaying long term stability (τ(approximately-greater-than)30 μs). The branching between predissociation and autoionization is explained by the effect of the dc electric field which mixes strongly the optically accessible p Rydberg series with the high l manifold beyond n=100. The long lifetimes observed experimentally indicate that ml mixing becomes important as soon as l mixing sets in. © 1995 American Institute of Physics.

Notes: The pulsed field ionization (PFI) zero-kinetic-energy (ZEKE) photoelectron spectrum of argon has been recorded in the region of the transition from the ground neutral state (1S0) to the first two ionization limits corresponding to the two spin–orbit levels (2P3/2 and 2P1/2) of the ground state of the ion. The high-n Rydberg states (85〈n〈200) belonging to the series converging to the upper spin–orbit state have a lifetime which is more than 50 times longer than expected for the optically accessible ns' and nd' series. A series of experiments with pulsed and continuous electric fields of different magnitude shows that the nature and the lifetimes of the high-n Rydberg states probed by ZEKE spectroscopy depend critically on the experimental conditions, in particular on electric field and collisional effects. New experimental results are presented which contribute to a better understanding of the mechanisms which lead to the formation of the unexpectedly long-lived states which are observed in ZEKE spectroscopy.