Michael C. Heaven

Rotational, fine, and hyperfine structure in the high‐resolution electronic spectrum of ArOH and ArOD

A number of vibrational bands of the A 2Σ+↔X 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high‐resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the A state with clear evidence for at least some electronic reorganization between Ar and the open‐shell OH radical in the complex. Conversely, the X state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed‐shell species.

Electronic spectroscopy of the ArOH and ArOD complexes

Laser induced fluorescence spectra of the ArOH bands associated with electronic excitation of the OH A–X(v’=0−v‘=0) transition were reported previously. An extensive search in the vicinity of the OH/OD A (2 Σ+ )−X(2 Π) system (v’=0−v‘=0 and v’=1−v‘=0) led to the observation of 36 vibronic bands which were identified as belonging to ArOH or ArOD complexes. All of the bands were found to originate from the 2 Π(3/2), v‘=0 ground electronic state of the OH/OD radical. Two distinguishable vibronic structures were identified. A lower energy vibrational progression was assigned to the van der Waals stretch. This band system was designated as the ‘‘A’’ bands. A higher energy set of bands exhibited a different vibrational structure which did not fit a simple vibrational mode progression. These bands were labeled as the ‘‘U’’ system. Spectra showing partially resolved rotational structure were recorded for all of the observed ArOH/ArOD bands. The A and U systems were also distinguished by differences in their rotat...

Spectroscopy of the AlAr van der Waals complex: Rotationally resolved B 2Σ+←X 2Π1/2 electronic transitions

Rotationally resolved spectra were recorded for six bands of the AlAr B 2Σ+←X 2Π1/2 transition. Vibrational and rotational constants derived from these spectra were used to determine the upper and lower state potential energy curves. The accuracy of these potentials was assessed through calculations of the spectroscopic constants and Franck–Condon factors. Dissociation energies of D’e=440+35−8 cm−1 and D′e=180+40−10 cm−1 were obtained for the B and X states, respectively. The interaction between X 2Π1/2 and the low‐lying A 2Σ+ state has been characterized by analysis of the ground‐state lambda doublet splitting.

Spectroscopy of metastable species in a free‐jet expansion: The D’←A’ transition of I2

The A’2u 3Π state of I2 is observed in a free‐jet expansion of I2 in Ar, where it is prepared by ArF laser irradiation. Laser excitation spectra are recorded for 37 bands in the υ’←0‘ progression of the D’2g(3P2)←A’ transition. The spectra display rotational temperatures of ∼5 K. At the 0.08 cm−1 resolution of the probe laser, rotational congestion near the band origins necessitates analysis by a nonlinear least‐squares contour simulation method. Subsequent correlated fits of the band‐by‐band results are combined with other results to produce global constants valid for υ’=0–86, υ‘=0–32.

Electronic spectroscopy and ionization potential of UO2 in the gas phase

The electronic spectroscopy of UO(2) has been examined using multiphoton ionization with mass-selected detection of the UO(2) (+) ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO(2) were observed in the range from 17,400 to 32,000 cm(-1). These bands originated from the U(5fphi(u)7ssigma(g))O(2) X (3)Phi(2u) and (3)Phi(3u) states. The stronger band systems are attributed to metal-centered 7p<--7s transitions. Threshold ionization measurements were used to determine the ionization potentials of UO and UO(2). These were found to be higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations.

Dissociation dynamics of I2(B)–Ar: Rotational population distributions of I2(B,v) fragments from the T-shaped and linear complexes

Optical-optical double resonance techniques have been used to examine the dissociation dynamics of I2(B)–Ar. Rotational population distributions were characterized for the I2(B,v) fragments. Vibrational predissociation of the T-shaped complex yielded fragments with smooth rotational distributions. The high-energy limits of the distributions were consistent with events that channeled almost all of the available energy into product rotation. These data indicate a dissociation energy for I2(B)–Ar of D0(C2v)=220 cm−1. Most initial states of the complex produced bimodal rotational population distributions, but a few gave Boltzmann-type product distributions. The dependence of the character of the distribution on the initially excited state suggests that predissociation is mediated by intramolecular vibrational energy redistribution. Dissociation of linear I2(B)–Ar yielded fragments with Boltzmann type rotational population distributions. Excitation of the complex within the bound regions of the B–X transition ...

Gain and lasing of optically pumped metastable rare gas atoms

Optically pumped alkali vapor lasers are currently being developed in several laboratories. The objective is to construct high-powered lasers that also exhibit excellent beam quality. Considerable progress has been made, but there are technical challenges associated with the reactivity of the metal atoms. Rare gas atoms (Rg) excited to the np(5)(n+1)s (3)P(2) configuration are metastable and have spectral properties that are closely similar to those of the alkali metals. In principle, optically pumped lasers could be constructed using excitation of the np(5)(n+1)p←np(5)(n+1)s transitions. We have demonstrated this potential by observing gain and lasing for optically pumped Ar(*), Kr(*) and Xe(*). Three-level lasing schemes were used, with He or Ar as the collisional energy transfer agent that established the population inversion. These laser systems have the advantage of using inert reagents that are gases at room temperature.

Spectroscopy of the ground and low-lying excited states of ThO+.

The ThO(+) cation is of interest as it is a useful prototype for experimental and theoretical studies of bonding in a simple actinide compound. Formally the ground state of ThO(+) has the configuration Th(3+)(7s)O(2-), where there is a single unpaired electron associated with a closed-shell Th(4+)-ion core. The first tier of excited states above the X (2)Sigma(+) ground state is expected to be 1 (2)Delta, 1 (2)Pi, and 2 (2)Sigma(+) derived from the Th(3+)(6d)O(2-) configuration. Spectroscopic observations of ThO(+) using the pulsed field ionization-zero kinetic-energy photoelectron technique are reported here. Rotationally resolved spectra were recorded for the X (2)Sigma(+), 1 (2)Delta, and 1 (2)Pi states. Extensive vibrational progressions were observed. Surprisingly, it was found that ionization of ThO decreases the dissociation energy, while increasing the vibrational frequency and decreasing the bond length. Accurate values for the ionization energies of ThO [53 253.8(2) cm(-1)] and Th [50 868.71(8) cm(-1)] were determined as part of this investigation.

Spectroscopy and relaxation dynamics of I2Arn clusters. Geminate recombination and cluster fragmentation

I2Arn clusters yield visible and near‐UV emissions when excited to the states which correlate with the first ion‐pair manifold of I2. These states may be accessed by 193 nm excitation of ground‐state clusters, or near‐UV excitation of electronically metastable I2(A’ or A)Arm. Comparisons of the cluster and I2/Ar matrix spectra suggest that such excitations result in fragmentation and ‘‘melting’’ of the clusters prior to emission. 532 nm photodissociation of I2 within the clusters is followed by geminate recombination which populates the A’, A, and X states. The probability for ejection of the recombined I2 from the clusters during the relaxation process appears to be size dependent. These results represent tentative steps towards the study of I(2P3/2)+I(2P3/2) recombination in Ar clusters. They also indicate that dissociation, recombination, and fragmentation processes may be used as a general method for generating metastable species in supersonic expansions.

Density functional calculations of beryllium clusters Ben, n=2–8

Neutral beryllium clusters Ben, na 2‐8, were investigated by density functional techniques. To minimize errors, geometry optimization, frequency and energy calculations were all carried out on the same level of theory, employing a large 6-311++G(3df) basis set. The method reproduces well the experimentally known bond length and vibrational frequency of the dimer, but its binding energy is still significantly overestimated. The computed trends of the vibrational frequencies, bond lengths and binding energies of the clusters as a function of the number of atoms are discussed. The binding energies are found to increase rapidly as a function of size, and approach the binding energy of the bulk metal, 54.1 kJ per bond. ” 2000 Elsevier Science B.V. All rights reserved.