Volume 94, Issue 12, 15 June 1991
Index of content:
94(1991); http://dx.doi.org/10.1063/1.460144View Description Hide Description
We calculate the signal in a time‐resolved, coherent light scattering experiment on a dilute solution of a nonpolar solute dissolved in a nonpolar solvent. We model the solute and solvent molecules as electronic two level systems, and assume that only the solute interacts with radiation. The electronic degree of freedom is treated perturbatively within the Born–Oppenheimer approximation. The nuclear degrees of freedom are treated classically, and their effects are modeled by a molecular dynamics simulation. We assume the electronic absorptionspectrum is dominated by inhomogeneous broadening. The validity of this assumption for our model is verified through the calculation of the absorption line shape with and without the inhomogeneous broadening assumption. A quantitative measure of the homogeneous dephasing time is obtained from a calculation of the photon echo signal for our model. The time dependence of the light scattering signal is shown to reflect both dynamics of the fluid in its electronic ground state and in its electronic excited state. We analyze the relative importance of contributions to the signal from these two types of dynamics as a function of experimental time scale and laser frequency.
Influence of molecular reorientational processes on the circular intensity difference scattering by optically active media in dc electric field94(1991); http://dx.doi.org/10.1063/1.460145View Description Hide Description
The influence of reorientational processes on circular intensity difference scattering (CIDS) in a static electric fieldE 0 is analyzed for systems of optically active dipolar molecules with the point group symmetries 3, 4, 6, and ∞ and of optically active nondipolar molecules with the symmetries 32, 422, and 622 for three experimental configurations: (i) at E 0 perpendicular to the plane of scattering; (ii) E 0 parallel to the incident light beam; and (iii) E 0 parallel to the wave scattered in the direction of observation. The effect is described in terms of molecular reorientation functions and is dependent on the experimental field configuration, the electro‐optical properties of the molecules, and the field strength E 0. Numerical calculations of molecular reorientation functions are performed for arbitrary degrees of electric saturation. The effects are accessible to observation in solutions of macromolecules by advanced laser and optoelectronic techniques.
Resonance‐enhanced multiphoton ionization‐photoelectron spectra of CO2. I. Photoabsorption above the ionization potential94(1991); http://dx.doi.org/10.1063/1.460146View Description Hide Description
Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant‐enhanced multiphoton ionization‐photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long‐lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above‐threshold ionization. This resonance‐enhanced above‐threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+ 2 ionic states at both the four‐ and five‐photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+ 2, with an ionization potential of 21.4 eV.
94(1991); http://dx.doi.org/10.1063/1.460147View Description Hide Description
The theory of Van der Waals complexes formed from atoms and open‐shell (Σ and Π) diatomic molecules is developed, paying particular attention to the quantum numbers that are conserved in the complex and the angular momentum coupling cases that may be observed. Complexes formed from diatoms in multiplet Σ states may exhibit several different coupling schemes closely analogous to Hund’s coupling cases for diatomic molecules. Complexes formed from diatoms in Π states usually exhibit a coupling scheme in which the (signed) projection P of the diatom angular momentumj onto the intermolecular axis is nearly conserved. Correlation diagrams showing the bending energy levels as a function of potential anisotropy are given for complexes containing diatomic molecules in both Σ and Π states. The transition from free internal rotor quantum numbers to near‐rigid bender quantum numbers with increasing anisotropy is investigated. The cases of Ar–OH and Ne–OH are considered as examples.
94(1991); http://dx.doi.org/10.1063/1.460148View Description Hide Description
We applied hydrostatic pressure to spectral holes burned into a resorufin doped ethanol/methanol glass. We found that the line shift is perfectly linear with pressure and showed a pronounced dependence on the burn frequency as predicted by theory [J. Chem. Phys. 9 0, 3274 (1989)]. We exploited the burn frequency dependence to determine the solvent shift of the dye probe and the compressibility of the alcohol glass used. On the other hand, the behavior of the hole width under pressure shows features not predicted by theory: The broadening is, like the line shift, dependent on the burn frequency within the inhomogeneous band, yet in a nonlinear fashion. We attribute the color effect in the pressure induced broadening of the hole to a breakdown of the Gaussian approximation.
94(1991); http://dx.doi.org/10.1063/1.460149View Description Hide Description
Unavoidable beam crossings within a spherical‐mirror, multipass stimulated Raman gain cell give rise simultaneously to forward‐ and backward‐scattering Raman signals. In the Doppler‐broadened/Dicke‐narrowed regime of density, the lineshape is a function of the momentum transfer in the scattering process and thus the observed spectra will have more complex lineshapes than those seen with simple forward or backward scattering geometries. The analyses necessary to quantitatively account for such forward–backward spectra are summarized. These spectra enable unique experimental tests of the lineshape functions used for the description of the Raman Q‐branch spectrum under conditions where Doppler contributions and Dicke narrowing are significant. Results for the D2:D2 and D2:He systems support the well‐known Galatry, or soft collision, lineshape function. However, in the case of D2:Ar, our results suggest the need to employ the more general, complex soft collision function. In addition, these studies have provided data on linear‐with‐density line broadening coefficients (previously published) and line shifting coefficients (reported here) for these molecular systems.
94(1991); http://dx.doi.org/10.1063/1.460150View Description Hide Description
A (2+1) resonance‐enhanced multiphoton ionization–photoelectron spectroscopy (REMPI‐PES) study of the OH radical has been carried out in the two‐photon energy region between 81 300 and 88 900 cm−1. Translationally and rotationally hot OH radicals are generated via photodissociation of hydrogen peroxide or formic acid. The known D 2Σ− (v’=0–2) and hitherto unobserved 3 2Σ−(v’=0) intermediate states in this region (at 81 815.8 and 87 643.7 cm−1 above the ground state) are shown to possess predominant Rydberg character. From the rotational structure in the REMPI spectrum physical parameters have been derived for these states.
94(1991); http://dx.doi.org/10.1063/1.460151View Description Hide Description
The lowest triplet state T 0 of the all‐t r a n s isomers of dodecapentaenal, decatetraenal, octatrienal, and hexadienal dissolved in polyethylene films has been investigated by electron–spin–echo spectroscopy in combination with pulsed laser excitation. Microwave transitions between the spin sublevels have been observed both in magnetic field and in zero field. For these polyenals T 0 is found to be of ππ* character. The direction of the principal axes of the fine‐structure tensors and the frequencies of the zero‐field transitions have been determined. The largest zero‐field splitting varies from 2496 MHz for dodecapentaenal to 5260 MHz for hexadienal. The zero‐field splitting turns out to be inversely proportional to the number of double bonds in the conjugated chain, which is found to be compatible with a description of the triplet excitation as the promotion of an electron from the highest occupied to the lowest unoccupied molecular orbital. The upper spin sublevel is preferentially populated in the intersystem crossing process and decays fastest with a lifetime of 14 μs for dodecapentaenal, of 28 μs for decatetraenal, and of 77 μs for octatrienal. This variation is adequately described by an energy‐gap law, which indicates that the Franck–Condon factors play a dominant role in the decay process.
Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes94(1991); http://dx.doi.org/10.1063/1.460152View Description Hide Description
We present a perturbation theoryanalysis of four time‐resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherencespectroscopy. The analysis shows that ground‐state rotational constants determine the rotational coherenceeffects in fully resonant, time‐resolved stimulated Raman‐induced fluorescence depletion (TRSRFD), excited‐state rotational constants determine such effects in time‐resolved stimulated emission spectroscopy (TRSES), and both ground‐ and excited‐state constants do so in time‐resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance‐enhanced shows that this method gives rise to qualitatively different rotational coherenceeffects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground‐state rotational coherencespectroscopy. Expressions for the calculation of rotational coherenceeffects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground‐ and excited‐state rotational coherenceeffects are manifest in TRFD traces.
94(1991); http://dx.doi.org/10.1063/1.460722View Description Hide Description
This paper details an investigation of the variation in the electronic transition moment with internuclear separation for the NO(B 2Π–X 2Π) transition. Measurements of the relative intensities of a number of NO B–X vibronic transitions having a common upper level were used to construct a relative transition‐moment function between 1.27 and 1.60 Å. After normalizing this relative function by experimentally determined radiative lifetimes, the transition‐moment function was extended down to 1.23 Å by incorporating data from oscillator strength measurements. In contrast to empirical transition‐moment functions that have been proposed previously, the function in this paper decreases with increasing internuclear separation. Unlike these other functions, however, this one is consistent with theoretical predictions, with most available oscillator strength data, and with the observed trend in B‐state radiative lifetimes as a function of vibrational level.
Picosecond–microsecond structural relaxation dynamics in polypropylene glycol: Impulsive stimulated light‐scattering experiments94(1991); http://dx.doi.org/10.1063/1.460153View Description Hide Description
Picosecond time‐resolved impulsive stimulated light‐scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature T g . Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz–5 GHz frequency range. In addition, time‐dependent thermal expansion is observed on nanosecond–microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent β=0.4 and with the average relaxation time given by the Vogel–Tamman–Fulcher form. Comparisons to Brillouin scattering and photoncorrelation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly faster dynamics for T>T g .
94(1991); http://dx.doi.org/10.1063/1.460154View Description Hide Description
Rotationally resolved spectra of the two vibronic bands 61 0 and 162 0 and a vibronic van der Waals band of the benzene–Ar2 cluster are presented, whose vibronic assignments are based on the analysis of their rotational structures. A fit to the rotational line positions in the symmetric top spectra yields an accurate set of rotational constants in the ground and the excited electronic state and the exact values for the band origins of the bands. From these values the spectral shift between corresponding cluster and monomer bands as well as the frequency of the van der Waals symmetric stretching vibration in the excited electronic state are precisely determined. The structure of the cluster is identified to be symmetric with one Ar atom located on the C 6 axis on each side of the benzene ring at a distance of 3.58 Å in the S 0 state and 3.52 Å in the S 1 state. These bond lengths exactly agree with our recent values for benzene–Ar. From the result that the bond lengths are equal for the dimer and the trimer we conclude that there is no Ar–Ar interaction through the intermediate benzene ring plane.
94(1991); http://dx.doi.org/10.1063/1.460155View Description Hide Description
The direct absorptionspectrum of benzene in a free jet has been measured in the 130–260 nm region (S 1, S 2, and S 3 states, Rydberg series, and the first ionization limit) using synchrotron radiation as a light source. The absolute molar extinction coefficients (ε) of benzene in jets have been determined by scaling measured free‐jet values to the known value in the vapor phase for a broadband at 200.1 nm in the S 2 state. The vibrational temperature for ν16 mode was estimated to be 185 K. The maximum value of ε of the S 1absorption system was found to be 1400 l mol−1 cm−1(spectral bandwidth=0.065 nm). A shoulder observed at 205.45 nm in the S 2absorption system is assigned to the S 2 origin, induced by pseudo‐Jahn–Teller distortion.
Microwave spectrum of alkali metal tetrahydroborate. I. Rotational transitions and molecular structure of NaBH4 in the ground vibrational state94(1991); http://dx.doi.org/10.1063/1.460156View Description Hide Description
The rotational spectrum of NaBH4 was observed in the millimeter‐wave region using a high temperature absorption cell. The observed spectrum of NaBH4 showed the pattern of a symmetric top molecule: Strong and weak for K=3n and 3n±1, respectively, because of the nuclear spin statistical weight for C 3v symmetry. The rotational and centrifugal distortion constants for the 11B and 10B species were determined. The observed rotational constants of Na11BH4 and Na10BH4, combined with the assumption that r(B–H b )−r(B−H t )=0.04 Å and θ (H b –B–H t )=111°, gave estimates for r(Na–B) and r(B–H b ) to be 2.308±0.006 Å and 1.28±0.10 Å, respectively, where the uncertainties are mainly due to those of the assumed values. This bond length obtained for Na–B is much shorter than the reported value in crystal: 3.08 Å. The bond lengths derived indicate that NaBH4 has a tridentate molecular structure with three bridging hydrogens. This result agrees with those of a b i n i t i o calculations.
High resolution laser spectroscopy up to the dissociation limit of the NaK B 1Π state, and predissociation near the dissociation limit94(1991); http://dx.doi.org/10.1063/1.460157View Description Hide Description
Doppler‐free high resolution spectrum of the B 1Π(v’,J’)–X 1Σ+(v‘,J‘) transitions of 23Na39K was measured by the optical–optical double resonance (OODR) polarizationspectroscopy. The transition lines up to the v’=43 level, which was estimated to be 1.8 cm−1 below the dissociation limit, were observed. The potential energy curve for internuclear distance from 3.25 to 15.6 Å was calculated by the RKR method, and the inverse‐power coefficient was determined by analyzing the long‐range RKR turning points. The B 1Π state dissociates to Na(3s 2 S 1/2)+K(4p 2 P 3/2) atoms without a potential hill near the dissociation limit. The dissociation energy was determined to be 1324.3±0.3 cm−1 from the LeRoy–Bernstein plots. Remarkable line broadenings were observed for transitions higher than the dissociation energy to Na(3s 2 S 1/2)+K(4p 2 P 1/2) atoms. This is identified as originating from the predissociation to Na(3s 2 S 1/2)+K(4p 2 P 1/2) atoms. The predissociation is shown to be caused by a spin–orbit interaction between the B 1Π and (2)3Σ+ states, and the potential curves are expected to cross around the inner turning point of the B 1Π (v’=34) level.
Methyl group torsional dynamics from rotationally resolved electronic spectra. 1‐ and 2‐methylnaphthalene94(1991); http://dx.doi.org/10.1063/1.460158View Description Hide Description
Rotationally resolved fluorescence excitation spectra of three vibronic bands in the S 1←S 0 transitions of 1‐ and 2‐methylnaphthalene (1 and 2MN) have been obtained. Each band exhibits perturbations that are produced by an interaction between the restricted torsional motion of the attached methyl group and the overall rotational motion of the entire molecule. A complete analysis of these effects yields values of the torsional barrier heights, the rotational constants, and the torsion–rotation perturbation coefficients of all vibronic levels that participate in the transitions. These values depend significantly on the position of the methyl group attachment, on the electronic state of the naphthalene chromophore, and on its vibrational state, as well. For example, V 3 (the threefold torsional barrier) decreases from 809 cm− 1 in 00 1MN to 128 cm−1 in 00 2MN. D (the largest first‐order torsion–rotation perturbation term) increases from 0.03 MHz in 00 1MN to 406 MHz in 00 2MN, a change of more than 4 orders of magnitude. The V 3 values of 00 and 8̄1 1MN are 563 and ≤ 373 cm−1, respectively. A full discussion of these dynamically relevant effects and their dependence upon both electronic and geometric factors is given.
The jet‐cooled fluorescence excitation spectrum and ring‐bending potential‐energy function and conformation of 2‐cyclopenten‐1‐one in the S 1(n,π*) electronic excited state94(1991); http://dx.doi.org/10.1063/1.460159View Description Hide Description
The jet‐cooled fluorescence excitation spectra of 2‐cyclopenten‐1‐one and its 5,5‐d 2 isotopomer have been recorded in the 370–340 nm region. The electronic origin for the undeuterated species occurs at 27 210 cm−1 for the S 1(n,π*) electronic excited state. The vibrational frequencies for the three carbonyl motions and the nine ring modes were observed for the excited state. Bands at 67, 158, and 256 cm−1 for the d 0 species, at 63, 147, and 240 cm−1 for the 5‐d 1 isotopomer, and at 59, 138, and 227 cm−1 for the d 2 species were assigned to the ring‐puckering motion in the S 1 state. A single one‐dimensional potential‐energy function accurately fits the data for all three isotopomers. This function is nearly purely quartic in character and shows the ring to be planar in the electronic excited state. However, it has become less rigid, and this is ascribed to a decrease in initial angle strain within the ring. The C=O and C=C stretching frequencies occur at 1418 and 1357 cm−1 for the d 0 molecule. The ring‐twisting frequency for the S 1 state occurs at 274 cm−1. Previous electronic absorption measurements had resulted in a misassignment for this motion.
94(1991); http://dx.doi.org/10.1063/1.460160View Description Hide Description
Employing a laser based time‐of‐flight mass spectrometer system, ion dip spectra for phenol (Ph), Ph(H2O)1, Ph(H2O)3, and Ph(H2O)4 were obtained in the range of 500–1300 cm−1 from a variety of initially pumped states. Dramatic enhancement of the signal‐to‐noise ratio of the cluster ion dip spectra relative to that of the bare phenol is attributable to the increase in the excited state singlet lifetime of the hydrated phenol chromophore. Several dips in the Ph(H2O)1spectrum exceed the ‘‘saturation’’ limit of 50%, indicating that significant relaxation of the downpumped ground state is occurring via low frequency vibrational modes of the H2O solvent ‘‘bath.’’ Excitation of the hydrogen bond stretch (σ 1 0=156 cm−1 ) in the S 1 state of the Ph(H2O)1 cluster reveals that the ground state (S 0) hydrogen bond stretch, σ 0 1, is 151(±1) cm−1, a mode which appears to be built off of phenol fundamental and combination bands. A second intermolecular band is also evident at 141(±2) cm−1. There is no evidence of an analogous wag mode when pumping the S 1 bend (β1 0), suggesting that the intermolecular modes in S 1 are highly coupled. Attempts to obtain ion dip spectra for Ph(H2O)2 went unrewarded, presumably due to the anomalously short S 1 lifetime of the Ph(H2O)2 cluster. Spectra for Ph(H2O)3 and Ph(H2O)4 were obtained which show prominent phenol bands, with low frequency (∼10 cm−1 ) progressions built off of these bands. The intermolecular hydrogen bond stretch for Ph(H2O)3 and Ph(H2O)4 in the ground state are 189(±1) cm−1 and 185(±1) cm−1, respectively.
94(1991); http://dx.doi.org/10.1063/1.460161View Description Hide Description
In this paper we advance and apply a molecular theory of pure radiative lifetimes τ r of (aromatic molecule)⋅(rare gas) n heteroclusters. The modification of τ r in the heterocluster, relative to the bare molecule value, originates from intermolecular interactions between molecular multicenter transition monopoles, which are described by π electron approximation, and the rare‐gas atom transition dipoles, which are specified in terms of the static atomic polarizability α. The calculated changes of τ r of 9,10 dichloroanthracene⋅A1 (A=Ar, Kr, Xe) heterodimers are in good agreement with experiment; the theory accounting for the nearly linear dependence of the change of τ r on α.
Calculations of the radiative lifetimes of 9,10 dichloroanthracene⋅Ar n (n=1–34) heteroclusters were performed using structural information from potential optimization for small (n=1–3) heteroclusters, static structural data for small and medium‐sized (n=1–8) heteroclusters, and (constant energy) finite‐temperature molecular dynamics simulations for medium‐sized and large (n=5–34) heteroclusters. The structural sensitivity of τ r is manifested by different values of τ r for distinct structural isomers, by the nonmonotonous size effects on τ r with increasing n, and by pronounced decrease of τ r with increasing temperature. For medium‐sized clusters (n=5–18), the temperature dependence of τ r originates from the enhancement of atomic motion on both sides of the aromatic microsurface and the occupation of the peripheral region, while for large clusters (n≂34) two layer–one layer isomerization processes with increasing temperature result in dramatic changes of τ r . Satisfactory overall agreement between theory and experiment for 9,10 dichloroanthracene⋅Ar n n=1–34 heteroclusters was accomplished.
The dominating 9,10 dichloroanthracene⋅Ar n structures for n=1–18, which yield agreement between theory and experimental for τ r , correspond to nearly equal distribution of the rare gases on both sides of the aromatic microsurfaces at T≤20 K, while large n=34 two‐layered structures in the temperature domain 22 K≤T≤40 K account for the experimental τ r result. τ r serves as a useful spectroscopic probe for the interrogation of the structure and isomerization dynamics in heteroclusters.
Vacuum ultraviolet emission spectra of the helium and neon alkali ions in the range between 60–80 nm94(1991); http://dx.doi.org/10.1063/1.460162View Description Hide Description
Mixtures of neon or helium with alkali vapor were excited using an argon ion beam. The emission continua observed in the vacuum ultraviolet region between 60 and 80 nm are assigned to the decay of the ionic excimers He+K, He+Na, He+Li, Ne+Rb, Ne+K, Ne+Na, and Ne+Li into the ground state consisting of a rare gas atom and an alkali ion. The binding energies of these excimers range between 0.46 and 0.85 eV. The obtained decay energies and the fine structure splittings agree with the predictions by calculations using phenomenological potentials and by a b i n i t i o calculations. Intense emission due to the inner shell excitation of K and Rb is also observed.