The effect of electronic resonances on the cross sections for Raman scattering from localized and... more The effect of electronic resonances on the cross sections for Raman scattering from localized and delocalized vibrational modes is considered for the cases where the resonance is with a localized electronic state and with a delocalized electronic state. It is concluded that resonance with a localized electronic state increases the Raman cross section of the localized mode significantly more than that of the delocalized mode while resonance with a delocalized electronic state has the opposite effect. Furthermore, a coupling of electronic states following a conformational change in a polymer is predicted to decrease the resonance Raman cross section of a localized vibrational mode. Consequently, observations of such effects will serve to identify local phonon modes in mixed molecular crystals and provide information about conformational change in biomolecules.
Recent studies of the Stark effect on bands in the laser induced phosphorescence excitation spect... more Recent studies of the Stark effect on bands in the laser induced phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum, believed to arise from the T2(nπ*) state, was the same within experimental uncertainty as that for the T1(ππ*) origin. This result may indicate that the T2(nπ*) state is strongly coupled to a set of nearby T1(ππ*) vibronic levels. In order to evaluate the effect of the density of ππ* vibronic levels on the dipole moment of the T2(nπ*) state, we have made similar measurements on 2,4- and on 2,5-dimethylbenzaldehyde. We have found that as the T1-T2 energy spacing decreases, the dipole moment of the second triplet state becomes more characteristic of an nπ* state.
The rotational distribution in the N2+ (B) state produced by core excitation and decay is reporte... more The rotational distribution in the N2+ (B) state produced by core excitation and decay is reported for three different core excitation energies. In contrast to the case of valence excitation, the rotational branching ratios do not exhibit a significant dependence on excitation energy. This similarity is attributed to the uniformly high kinetic energies of the emitted electron for the three cases and provides evidence for the production of singly charged molecular ions in the core ionization continuum through an interchannel coupling mechanism.
International Journal of Mass Spectrometry and Ion Physics, 1983
A brief review is given of some of the work at the National Bureau of Standards using photon stim... more A brief review is given of some of the work at the National Bureau of Standards using photon stimulated desorption of ions from adsorbed molecules. Ion desorption mechanisms for ionically and covalently bonded absorbates are discussed. Selected results are presented for O on Ti(001), Nb(001) and W(111), CO on Ru(001), and H2O, CH3OH and C6H12 on Ti(001).
The use of Stark spectroscopy to measure the space-charge distribution in insulators, semiconduct... more The use of Stark spectroscopy to measure the space-charge distribution in insulators, semiconductors, and photoconductors is described and illustrated with experimental results for an organic photoconductor. The observed charge distribution is understood in terms of the variation of charge carrier generation and trapping through the crystal.
Author Institution: California Institute of Technology^{1}$. ${^{1}}$Silbey, Jortner, Vala, and R... more Author Institution: California Institute of Technology^{1}$. ${^{1}}$Silbey, Jortner, Vala, and Rice, J. Chem. Phys. 42, 2948 (1965).From the experiments on the $^{1}B_{2a}$ state of naphthalene, naphthalene-$d_{8}$, and their dilute mixed crystals, values for $M_{12}$ and $M_{1} (i = a,b,c)$ are deduced where $M_{12}$ represents the interaction between interchange equivalent molecules and the $M_{1}$ represent the interaction between the translationally equivalent molecules along the a,b, and c crystal axes. The quasiresonance shift is found to be $12.3\pm 4 cm^{-1}$ compared to the $147\pm 6 cm^{-1}$ resonance splitting. The translational $(k =0)$ shift is found to be $5\pm 7 cm^{-1}$. Thus, $M_{12}= 18.4 \pm 0.4 cm^{-1}$ and $M_{a}+M_{b}+M_{e} = 2.5\pm 3.5 cm^{-1}$. The site (static) shift is $466\pm 10 cm^{-1}$. The common observation of very narrow lines in the exciton (Davydov) splitting of molecular crystals is not due to the discreteness of exciton levels but is due to the spectroscopic selection rule $(\Delta k = 0)$ and the fact that all molecules are in the ground state $(k = 0)$ at the low temperatures usually employed. By observing transitions between vibrational and electronic exciton states, the complete electronic k band structure is observed. For naphthalene, the observed band agrees with the pure and mixed crystal data reported above. For benzene, it agrees with the location of the forbidden exciton component reported in the 1965 Symposium. Both of the above experiments indicate that $M_{12}\gg M_{1}$. Noting that $M_{12}$ and $M_{1}$ represent interactions between nearly perpendicular and parallel molecules, respectively, this fact may indicate that intramolecular $\sigma-\pi$ mixing should be considered in the calculation of the intermolecular interactions. The quasiresonance data seem to be inconsistent with a recent prediction of the energy of the charge-transfer state in crystalline $naphthalene
(1) E. Migirdicyan, Chem. Phys. Letters, 12, 473, (1972)Author Institution:The electric field eff... more (1) E. Migirdicyan, Chem. Phys. Letters, 12, 473, (1972)Author Institution:The electric field effect on several of the bands in the phosphorescence excitation spectrum of 2, 4, 5-trimethylbenzaldehyde, isolated in a durene single crystal at 4.2$^{\circ} K$ , was measured. While the weak $T_{1}$ origin at 4139 {\AA} was narrow enough (ea. $1 cm^{-1}$) to resolve the stark splitting, the intense bands in the region previously assigned(1) to the second triplet state near 4060 {\AA} were broad (ca. $10 cm^{-1}$). A selective laser excitation technique was developed to overcome the problem of broad lines and determine the stark splitting of these bands. It was found that the most intense peak in the 4060 {\AA} region has the same $\Delta\mu$ vector, within the measurement uncertainties, as the $\tau_{1}$ origin. This is an unexpected result since the first triplet state is known to be $\pi\pi^{*}$ in character while the second is thought to be $n\pi^{*}$
1. Irwin C. Winkler and David M. Hanson, J. Am. Chem, Soc., 103, 6264 (1981)Recent studies of the... more 1. Irwin C. Winkler and David M. Hanson, J. Am. Chem, Soc., 103, 6264 (1981)Recent studies of the electric field effect on several bands of the laser induced phosphorescence excitation spectrum of 2, 4, 5-trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum was the same, within experimental uncertainty, as that for the $T_{1}(n\pi\ast)$ phosphorescence $origin^{1}$. This result seems to indicate that either the assignment of this feature to the $T_{2}(n\pi\ast)$ state is incorrect or that the $T_{2}(n\pi\ast)$ state is strongly coupled to a set of $T_{1}(n\pi\ast)$ vibronic levels. In order to determine the origin and generality of this result, we have mesured the electric field effect on the phosphorescence excitation spectra of 2, 4 and 2, 5-dimethylbenzaldehyde. The results of these experiments will be presented and compared with those for trimethylbenzaldehyde
Journal of Electron Spectroscopy and Related Phenomena, 1995
Several electronic states of NH32+ were prepared by Auger decay following core electron ionizatio... more Several electronic states of NH32+ were prepared by Auger decay following core electron ionization of NH3. The ionic products from these states were observed by single-event energy-resolved Auger electron multiple-ion coincidence time-of-flight mass spectrometry. The dependence of the fragmentation channel branching ratios on the electronic state of the dication correlates with the energy threshold for each channel. In general, the
The use of a Stark switching technique to observe triplet state (T1) excitation energy transfer b... more The use of a Stark switching technique to observe triplet state (T1) excitation energy transfer between p-methoxybenzaldehyde (MBA) molecules in a p-dimethoxybenzene host crystal at 4.2 K is described. The results indicate that T1 is a ππ* state and that the rate of energy transfer is very slow, only 3 s−1.
The electronic Stark effect on the S1←S0 and T1→S0 transitions of quinoxaline in durene and napht... more The electronic Stark effect on the S1←S0 and T1→S0 transitions of quinoxaline in durene and naphthalene host crystals is used to demonstrate how the problem of space charge can be eliminated, to reveal the inadequacy of the Lorentz local field approximation, and to demonstrate a technique that provides critical information about the sign of the dipole moment change in centrosymmetric crystals.
The symmetry of excited states reached by photoexcitation of K-shell electrons of N2 and O2 is id... more The symmetry of excited states reached by photoexcitation of K-shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x-ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.
Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon a... more Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time-of-flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F2+ ions following fluorine 1s excitation in SiF4 provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two-hole final state that persists on the time scale of fragmentation. In contrast, no F2+ was observed for CF4, which indicates that fragmentation for this molecule is more characteristic of a delocalized two-hole state.
The native oxide, thermally oxidized and hydrofluoric acid rinsed surfaces of Si(100) have been c... more The native oxide, thermally oxidized and hydrofluoric acid rinsed surfaces of Si(100) have been characterized by photon stimulated ion desorption (PSD), and both photoelectron and Auger electron spectroscopies. The only species detected by PSD were H+ ions with different kinetic energies. Low kinetic energy H+ ions were detected only from the HF rinsed surfaces presumably arising from scission of Si—H bonds while higher kinetic energy ions attributed to adsorbed hydrocarbon dissociation were observed for all of the surfaces.
The effect of electronic resonances on the cross sections for Raman scattering from localized and... more The effect of electronic resonances on the cross sections for Raman scattering from localized and delocalized vibrational modes is considered for the cases where the resonance is with a localized electronic state and with a delocalized electronic state. It is concluded that resonance with a localized electronic state increases the Raman cross section of the localized mode significantly more than that of the delocalized mode while resonance with a delocalized electronic state has the opposite effect. Furthermore, a coupling of electronic states following a conformational change in a polymer is predicted to decrease the resonance Raman cross section of a localized vibrational mode. Consequently, observations of such effects will serve to identify local phonon modes in mixed molecular crystals and provide information about conformational change in biomolecules.
Recent studies of the Stark effect on bands in the laser induced phosphorescence excitation spect... more Recent studies of the Stark effect on bands in the laser induced phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum, believed to arise from the T2(nπ*) state, was the same within experimental uncertainty as that for the T1(ππ*) origin. This result may indicate that the T2(nπ*) state is strongly coupled to a set of nearby T1(ππ*) vibronic levels. In order to evaluate the effect of the density of ππ* vibronic levels on the dipole moment of the T2(nπ*) state, we have made similar measurements on 2,4- and on 2,5-dimethylbenzaldehyde. We have found that as the T1-T2 energy spacing decreases, the dipole moment of the second triplet state becomes more characteristic of an nπ* state.
The rotational distribution in the N2+ (B) state produced by core excitation and decay is reporte... more The rotational distribution in the N2+ (B) state produced by core excitation and decay is reported for three different core excitation energies. In contrast to the case of valence excitation, the rotational branching ratios do not exhibit a significant dependence on excitation energy. This similarity is attributed to the uniformly high kinetic energies of the emitted electron for the three cases and provides evidence for the production of singly charged molecular ions in the core ionization continuum through an interchannel coupling mechanism.
International Journal of Mass Spectrometry and Ion Physics, 1983
A brief review is given of some of the work at the National Bureau of Standards using photon stim... more A brief review is given of some of the work at the National Bureau of Standards using photon stimulated desorption of ions from adsorbed molecules. Ion desorption mechanisms for ionically and covalently bonded absorbates are discussed. Selected results are presented for O on Ti(001), Nb(001) and W(111), CO on Ru(001), and H2O, CH3OH and C6H12 on Ti(001).
The use of Stark spectroscopy to measure the space-charge distribution in insulators, semiconduct... more The use of Stark spectroscopy to measure the space-charge distribution in insulators, semiconductors, and photoconductors is described and illustrated with experimental results for an organic photoconductor. The observed charge distribution is understood in terms of the variation of charge carrier generation and trapping through the crystal.
Author Institution: California Institute of Technology^{1}$. ${^{1}}$Silbey, Jortner, Vala, and R... more Author Institution: California Institute of Technology^{1}$. ${^{1}}$Silbey, Jortner, Vala, and Rice, J. Chem. Phys. 42, 2948 (1965).From the experiments on the $^{1}B_{2a}$ state of naphthalene, naphthalene-$d_{8}$, and their dilute mixed crystals, values for $M_{12}$ and $M_{1} (i = a,b,c)$ are deduced where $M_{12}$ represents the interaction between interchange equivalent molecules and the $M_{1}$ represent the interaction between the translationally equivalent molecules along the a,b, and c crystal axes. The quasiresonance shift is found to be $12.3\pm 4 cm^{-1}$ compared to the $147\pm 6 cm^{-1}$ resonance splitting. The translational $(k =0)$ shift is found to be $5\pm 7 cm^{-1}$. Thus, $M_{12}= 18.4 \pm 0.4 cm^{-1}$ and $M_{a}+M_{b}+M_{e} = 2.5\pm 3.5 cm^{-1}$. The site (static) shift is $466\pm 10 cm^{-1}$. The common observation of very narrow lines in the exciton (Davydov) splitting of molecular crystals is not due to the discreteness of exciton levels but is due to the spectroscopic selection rule $(\Delta k = 0)$ and the fact that all molecules are in the ground state $(k = 0)$ at the low temperatures usually employed. By observing transitions between vibrational and electronic exciton states, the complete electronic k band structure is observed. For naphthalene, the observed band agrees with the pure and mixed crystal data reported above. For benzene, it agrees with the location of the forbidden exciton component reported in the 1965 Symposium. Both of the above experiments indicate that $M_{12}\gg M_{1}$. Noting that $M_{12}$ and $M_{1}$ represent interactions between nearly perpendicular and parallel molecules, respectively, this fact may indicate that intramolecular $\sigma-\pi$ mixing should be considered in the calculation of the intermolecular interactions. The quasiresonance data seem to be inconsistent with a recent prediction of the energy of the charge-transfer state in crystalline $naphthalene
(1) E. Migirdicyan, Chem. Phys. Letters, 12, 473, (1972)Author Institution:The electric field eff... more (1) E. Migirdicyan, Chem. Phys. Letters, 12, 473, (1972)Author Institution:The electric field effect on several of the bands in the phosphorescence excitation spectrum of 2, 4, 5-trimethylbenzaldehyde, isolated in a durene single crystal at 4.2$^{\circ} K$ , was measured. While the weak $T_{1}$ origin at 4139 {\AA} was narrow enough (ea. $1 cm^{-1}$) to resolve the stark splitting, the intense bands in the region previously assigned(1) to the second triplet state near 4060 {\AA} were broad (ca. $10 cm^{-1}$). A selective laser excitation technique was developed to overcome the problem of broad lines and determine the stark splitting of these bands. It was found that the most intense peak in the 4060 {\AA} region has the same $\Delta\mu$ vector, within the measurement uncertainties, as the $\tau_{1}$ origin. This is an unexpected result since the first triplet state is known to be $\pi\pi^{*}$ in character while the second is thought to be $n\pi^{*}$
1. Irwin C. Winkler and David M. Hanson, J. Am. Chem, Soc., 103, 6264 (1981)Recent studies of the... more 1. Irwin C. Winkler and David M. Hanson, J. Am. Chem, Soc., 103, 6264 (1981)Recent studies of the electric field effect on several bands of the laser induced phosphorescence excitation spectrum of 2, 4, 5-trimethylbenzaldehyde, isolated in a durene single crystal, revealed that the dipole moment associated with the most intense feature in the spectrum was the same, within experimental uncertainty, as that for the $T_{1}(n\pi\ast)$ phosphorescence $origin^{1}$. This result seems to indicate that either the assignment of this feature to the $T_{2}(n\pi\ast)$ state is incorrect or that the $T_{2}(n\pi\ast)$ state is strongly coupled to a set of $T_{1}(n\pi\ast)$ vibronic levels. In order to determine the origin and generality of this result, we have mesured the electric field effect on the phosphorescence excitation spectra of 2, 4 and 2, 5-dimethylbenzaldehyde. The results of these experiments will be presented and compared with those for trimethylbenzaldehyde
Journal of Electron Spectroscopy and Related Phenomena, 1995
Several electronic states of NH32+ were prepared by Auger decay following core electron ionizatio... more Several electronic states of NH32+ were prepared by Auger decay following core electron ionization of NH3. The ionic products from these states were observed by single-event energy-resolved Auger electron multiple-ion coincidence time-of-flight mass spectrometry. The dependence of the fragmentation channel branching ratios on the electronic state of the dication correlates with the energy threshold for each channel. In general, the
The use of a Stark switching technique to observe triplet state (T1) excitation energy transfer b... more The use of a Stark switching technique to observe triplet state (T1) excitation energy transfer between p-methoxybenzaldehyde (MBA) molecules in a p-dimethoxybenzene host crystal at 4.2 K is described. The results indicate that T1 is a ππ* state and that the rate of energy transfer is very slow, only 3 s−1.
The electronic Stark effect on the S1←S0 and T1→S0 transitions of quinoxaline in durene and napht... more The electronic Stark effect on the S1←S0 and T1→S0 transitions of quinoxaline in durene and naphthalene host crystals is used to demonstrate how the problem of space charge can be eliminated, to reveal the inadequacy of the Lorentz local field approximation, and to demonstrate a technique that provides critical information about the sign of the dipole moment change in centrosymmetric crystals.
The symmetry of excited states reached by photoexcitation of K-shell electrons of N2 and O2 is id... more The symmetry of excited states reached by photoexcitation of K-shell electrons of N2 and O2 is identified from the fragmentation anisotropy. For each molecule, ion yield excitation spectra were obtained at 90°, 54.7°, and 0° relative to the axis of soft x-ray polarization. The degree of polarization of the synchrotron radiation was determined experimentally. From this determination and the angular dependence of the ion yield, the photoabsorption anisotropy parameter β is obtained as a function of photon energy. Variations in the β spectra are discussed in terms of previously assigned spectral features and are compared with other experimental results. A comparison also is made with theoretical calculations, which are available for the case of N2.
Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon a... more Monochromatic synchrotron radiation was used to excite selectively core electrons of the carbon and fluorine atoms in carbon tetrafluoride and silicon and fluorine in silicon tetrafluoride. The fragmentation processes were examined using time-of-flight mass spectroscopy. The mass spectra show the distribution of ions collected in coincidence with low and high energy electrons. Distinct changes in the mass spectra with atomic site of excitation and photon energy are observed. The observation of F2+ ions following fluorine 1s excitation in SiF4 provides significant evidence for a ‘‘valence bond depopulation’’ mechanism involving the formation of a localized, two-hole final state that persists on the time scale of fragmentation. In contrast, no F2+ was observed for CF4, which indicates that fragmentation for this molecule is more characteristic of a delocalized two-hole state.
The native oxide, thermally oxidized and hydrofluoric acid rinsed surfaces of Si(100) have been c... more The native oxide, thermally oxidized and hydrofluoric acid rinsed surfaces of Si(100) have been characterized by photon stimulated ion desorption (PSD), and both photoelectron and Auger electron spectroscopies. The only species detected by PSD were H+ ions with different kinetic energies. Low kinetic energy H+ ions were detected only from the HF rinsed surfaces presumably arising from scission of Si—H bonds while higher kinetic energy ions attributed to adsorbed hydrocarbon dissociation were observed for all of the surfaces.
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Papers by David Hanson