W. C. Walker

Time-resolved excitonic luminescence processes in poly(phenylenevinylene)

Integrated luminescence spectra, time-resolved luminescence spectra, and luminescence decay times of stretch-oriented poly(phenylenevinylene) (PPV) are studied in the temperature range 300∼12 K. The reflectance spectrum at room temperature is also studied. The luminescence spectrum is composed of vibronic bands and a broad band, which are interpreted as caused by radiative annihilation of free and selftrapped excitons, respectively. Temperature dependence of the intensity of free-exciton luminescence and temperature dependence of the decay time are interpreted in terms of relaxation of excitons. The height of a potential barrier, which separates the free exciton state and the self-trapped state, is found to be 350 cm -1 . At 20 K, exciton relaxation toward the self-trapped state occurs by a quantum-mechanical tunneling process. The tunneling rate obtained is (90 ps) -1 .

Emission spectrum of rare gas dimers in the vacuum uv region. I. Ar2

Emission spectra of the neon dimer have been studied in the VUV region. Liquid nitrogen cooled neon was excited with a Tesla coil or transformer discharge. Three band systems I, II, and III were observed in the 735–1000 A range and classified as produced by electronic transitions to the common ground state X0+g(1S0) from the three low lying excited states 1u(3P2), 0+u(3P1), and 0+u(1P1), respectively. Evidence for the existence of potential humps in the lowest two upper states were observed and the hump heights were calculated to be 142±10 and 670±50 cm−1, respectively. Observations of additional spectra, a newly observed emission continuum and two diffuse band groups, are presented.

Photoluminescence of ArO and KrO in doped rare‐gas matrices

Photoluminescent emission and excitation spectra of matrices of Ar and Kr doped with O2 and N2O and excited by uv radiation between 200 and 110 nm are reported. To our knowledge, the normalized excitation spectrum of Ar:O2 provides the first measurement of the relative quantum yield for the photodissociation of O2 into O(3P)+O(1S). The differences between matrix isolated XeO, ArO, and KrO spectra are discussed. A simple model to explain the results is proposed.

Matrix interactions with rare‐gas oxide excimers

Matrix interactions of rare‐gas oxide molecules formed in solid matrices were studied by photoluminescence. Excitation energies between 8–12 eV were provided by both synchrotron radiation at the Stanford Synchrotron Radiation Laboratroy (SSRL) and a hydrogen discharge source at UCSB. Photoluminescence emission and excitation spectra of Ar:CO2(1%), Kr:CO2(1%), Kr:N2O(1%), and Kr:O2(1%) mixtures were obtained. Radiative lifetimes of ArO and KrO emissions from CO2 doped Ar and Kr matrices were also measured. The direct photodissociative threshold of CO2 to CO(X 1S+)+O(1S) in an Ar matrix was found to be 10.6 eV, in agreement with that of the gas phase. Further unambiguous evidence for radiationless, dissociative energy transfer, between the matrix free exciton and CO2, probably by a harpooning mechanism, was seen in Kr:CO2 below the direct photodissociation threshold. Temperature, matrix annealing, and character of the photofragment were found to have significant effects on the rare‐gas oxide molecular bindi...

Vacuum ultraviolet absorption spectra of solid N2O and CO2 at 53 K

Photoabsorption spectra of thin films of solid N2O and CO2 were obtained in the 105–240 nm wavelength region. The gas phase absorption continua were shifted by as much as 0.53 eV in the solid. Dipole forbidden transitions such as N2O(1Δ←1Σ+), CO2(1Δu←1Σ+g), and CO2(1Πg←1Σ+g) were intensified. The spin forbidden N2O(3Σ+←1Σ+) and N2O(3Δ←1Σ+) transitions were not detected even in 30 μm thick films. Dipole allowed transitions such as N2O(1Π←1Σ+), N2O (1Σ+←1Σ+), CO2(1Σ+u←1Σ+g), and CO2 (1Πu←1Σ+g) became diffuse. Just below the CO2 (1Σ+u←1Σ+g) absorption edge, an exciton peak was observed at 10.73 eV.

Polarization of three vacuum-ultraviolet monochromators measured with a biotite polarizer

The effective transmittance and the degree of polarization of two types of reflection polarizers, one a pyramidal type and the other a parallel type, are compared. A pyramidal polarizer was used to measure the polarization of three different monochromators in the region 1000–2000 A. The exit beam from two near-normal-incidence monochromators, a McPherson 225 and a McPherson 218, was not significantly polarized, while that from a Jarrell–Ash Seya–Namioka monochromator was about 35% polarized at 1000 A.

Vacuum ultraviolet absorption spectra of solid OCS at 53 K

Photoabsorption spectra of thin films of solid OCS were obtained in the 110–300 nm wavelength region. The absorption intensities and excitation energies of the 1Δ←1Σ+ and 1Π←1Σ+ transitions were changed in the solid, and higher energy Rydberg absorption was reduced and diffuse. Three absorption continua not normally observed in the gas phase were found at 4.3, 4.7, and 6.9 eV and assigned to the 3Σ+←1Σ+, 1Δ←1Σ+, and 3Π←1Σ transitions, respectively.

Photoabsorption of solid carbon dioxide from 7 to 12 eV

The photoabsorption spectra of solid and gas phase carbon dioxide were measured in the region from 7 to 12 eV. Transition energies were found to be shifted in the solid, and one forbidden transition was anomalously enhanced relative to its gas phase counterpart.

Exciton and Interband Spectra of Crystalline MnF 2

Reflection and absorption spectra of MnF2 (crystals were kindly supplied by F. Legend, Center for Nuclear Studies at Saclay, France, and V. Saccarino, University of California, Santa Barbara, California) in the region from 4.5 to 31 eV were investigated at room and liquid-nitrogen temperatures. Peaks associated with the onset of allowed transitions are explained in terms of excitons at Γ (10.48 and 10.55 eV) and at X (10.24 eV) and on this basis some features of the band structure are proposed. Splitting of these peaks is interpreted in terms of the spin–orbit splitting of p-like valence bands arising primarily from F− ions. The energy-loss spectrum contains peaks corresponding to plasma frequencies of electrons in half-filled d-like bands and in F− valence bands. Transitions over the range from 4.5 to 9.5 eV are ascribed to d excitons associated with states 6S(d5) → 4F(d5), 6D(d4s), and 4D(d4s) of the Mn2+ ion in an orthorhombic crystal-ligand field.

Photodissociative production of O(1S) from N2O and S(1S) from OCS in rare gas matrices

Photoluminescence excitation spectra of Ar:N2O (1%), Ar:OCS (1%), and Kr:OCS (1%) were obtained in the region 180–104 nm, using UV synchrotron radiation at SSRL. Photodissociation energy thresholds in the matrix were found to be: 8.06 eV for N2O→N2(X 1Σ+)+0(1S) and 7.17 eV for OCS→CO (X 1Σ+)+S(1S). Our results show several effects due to interactions between the matrix and the triatomic molecule: (1) differential blue shifts of the 1Π←1Σ+ and 1Σ+←1Σ+ transitions, (2) a greatly enhanced predissociation rate from 1Π to 1Σ+, (3) matrix dependent energy shifts of a particular Rydberg state, (4) nonradiative energy transfer between the matrix and the triatomic, involving either self‐trapped excitons and a Foster–Dexter mechanism or free excitons and a harpooning mechanism. To our knowledge, this is the first observation of the free exciton–harpooning mechanism in a rare gas matrix.