Volume 98, Issue 4, 15 February 1993
Index of content:
98(1993); http://dx.doi.org/10.1063/1.464185View Description Hide Description
Laser‐induced fluorescence is used to obtain the second overtone spectrum of ArHF. The method exploits intracavity circulating power of a Ti–sapphire ring laser to pump the weakly bound complex generated in a supersonic slit jet from v=0 to v=3. Fundamental (Δv=−1) emission is monitored using an infrared PbS detector. Intense fluorescence allows recording of the rotationally resolved sub‐Doppler spectra of (3000)←(0000), (3100)←(0000), and (3110)←(0000) transitions. We determine vibrational band origins of ν0=11 339.034 cm−1, 11 412.438 cm−1, 11 422.378 cm−1 and rotational constants of B=0.103 30 cm−1, 0.102 76 cm−1, 0.101 18 cm−1 for the (3000), (3100), and (3110) bands, respectively. Both the band origins and the rotational constants indicate that the weak Ar–HF van der Waals bond is strengthened as the HF stretch is vibrationally excited to higher states. All the observations are in near perfect accord with extrapolations of related constants in the HF stretching states of v=0–2.
Characteristic properties of the nuclear magnetic resonance–paramagnetic relaxation enhancement arising from integer and half‐integer electron spins98(1993); http://dx.doi.org/10.1063/1.464186View Description Hide Description
The influence of zero field splitting (zfs) interactions on the magnetic field dispersion profile of the nuclear magnetic resonance–paramagneticrelaxation (NMR–PRE) (i.e., the enhancement of nuclear magnetic relaxation rates that is produced by paramagnetic solute species in solution) has been explored systematically for S=1, 3/2, 2, and 5/2 spin systems using recently developed theory. To facilitate comparison of results for different spin values, the theory was expressed in a reduced form with Larmor frequencies in units of ω D (the uniaxial zfs parameter D in rad s−1), and correlation times and spin relaxation times in units of ω D −1. For S=1, the functional form of the profile can be described in terms of five types of qualitative features. Two of these are characteristic of Zeeman‐limit [Solomon, Bloembergen, and Morgan (SBM)] theory and result from the magnetic field dependence of the spin energy level splittings. The remaining three have no analog in Zeeman‐limit theory and arise from a change in the quantization axis of the electron spin precessional motion which, in the zfs limit, lies along molecule‐fixed coordinate axes, and, in the Zeeman limit, lies along the external field direction.
The reduced field dispersion profiles for the integer spin systemsS=1 and S=2 were found to be very similar to each other, the principal difference being that the midfield positions of the requantization features (types 2, 3, and 4) are shifted for S=2 relative to S=1, the magnitude and sign of the shift depending on the position of the nuclear spin in the molecular coordinate frame. For half‐integer spins, the dispersion profiles exhibit, in addition to the five features characteristic of integer spins, a sixth type of feature, which is centered somewhat to low field of ω S τ c =1, where τ c is the dipolar correlation time. The type‐6 feature results from field‐dependent level splitting of the m S =±1/2 Kramers doublet. It is present when ω D τ c ≥1. These theoretical predictions have been examined by means of reinterpretations of the NMR–PRE data for tris‐(acetylacetonato)–metal complexes of V(III) (S=1), Cr(III) (S=3/2), Mo(III) (S=3/2), Mn(III) (S=2), and Fe(III) (S=5/2). As predicted, type‐6 features are absent for the integer spin complexes, for which the T 1 field dispersion profiles are nearly field independent.
The experimental profiles were successfully simulated quantitatively by the generalized theory, but not by Zeeman‐limit theory. For the half‐integer spin systems, the predicted zfs‐related type‐6 features appear to be present in the profiles, particularly for Mo(acac)3, for which the data deviate significantly from the Zeeman‐limit profile in a manner that is explained by the generalized theory.
98(1993); http://dx.doi.org/10.1063/1.464187View Description Hide Description
Intracluster vibrational redistribution (IVR) and vibrational predissociation (VP) dynamics of 4‐ethylaniline (Ar)1, (N2)1, and (CH4)1 clusters have been studied by time‐correlated single photon counting, mass‐resolved excitation spectroscopy, and dispersed emission spectroscopy. The 4‐ethylaniline molecule has a low frequency ethyl group torsion vibrational mode, which is similar in energy to the van der Waals modes of the clusters. This mode, because of its low energy (∼35 cm−1), plays a role in the vibrational dynamics of the clusters studied. The cluster dissociation rates and product state distributions can be modeled by a serial IVR/VP mechanism for which the VP step is treated by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The resulting agreement between the calculated and experimental rates and product state intensities indicates that a statistical distribution of energy among all low frequency modes exists for 4‐ethylaniline/polyatomic solvent clusters in which k IVR≫k VP. For 4‐ethylaniline (Ar)1 clusters k VP≳k IVR and a statistical distribution of energy among the chromophore and van der Waals modes is not achieved. The central determining factor for the vibrational dynamics of these clusters is overall density of low energy modes.
Dynamics of third‐order nonlinearity of canthaxanthin carotenoid by the optically heterodyned phase‐tuned femtosecond optical Kerr gate98(1993); http://dx.doi.org/10.1063/1.464136View Description Hide Description
It is shown that polarization sensitive (optically heterodyned) detection combined with π/2 optical phase biasing between the optical Kerr gate signal and the local oscillator signal provides an effective method for the investigation of dynamics of third‐order nonlinear optical processes. The method permits selective enhancement of either the real or the imaginary component of the nonlinear optical response of the medium under study. Both the magnitudes and the signs of the real and the imaginary components of the complex third‐order optical susceptibility, contributing to the Kerr gate phenomenon, can thus be determined independently. The method is used to investigate the third‐order nonlinear optical response of solutions of canthaxanthin (4,4’‐dioxo‐β‐carotene) in tetrahydrofuran. With the use of ultrafast 60 fs laser pulses at 620 nm, the instantaneous, coherent part of the response is separated from an incoherent, time‐delayed response. The real and the imaginary components of the instantaneous complex second hyperpolarizability γ are derived. The analysis of the delayed part of the signal suggests that it originates in an effective χ(5) nonlinear process due to the change of linear susceptibility after population of a two‐photon excited state. However, an unusual feature observed is that the two‐photon pumped excited state does not produce a significant change of linear susceptibility, but it is a lower state, most probably the 2 1 A g state, subsequently populated by nonradiative relaxation which produces a dominant change in linear susceptibility responsible for the incoherent nonlinear optical response. From a theoretical fit of the temporal behavior of the delayed nonlinear response, the rates of nonradiative population and subsequent decay of the 2 1 A g state are derived. These rates are in qualitative agreement with published spectroscopic results for this class of compounds.
98(1993); http://dx.doi.org/10.1063/1.464137View Description Hide Description
Fragmentation of N2O after selective core excitation of terminal and central nitrogen, has been studied by a multicoincidence technique (PEPIPICO) using synchrotron radiation. We show that dissociation dynamics is dependent upon the excited site, especially in the case of the ‘‘atomization’’ of the molecule. The central nitrogen is always found with very little kinetic energy, except after Nt→Π* transition. A bent intermediate geometry is proposed to explain this observation.
98(1993); http://dx.doi.org/10.1063/1.464138View Description Hide Description
The IR fluorescence data of SF6 excited in its quasicontinuum (QC), reported in paper I [C. Angelié, J. Chem. Phys. 96, 8072 (1992)], are analyzed in comparison with IR multiphoton absorption (IRMPA) data, either in the nanosecond or in the picosecond regimes. A general description of the QC transition matrix elements is first derived. All the following features must be taken into account: multiple rovibrational transitions, perturbative redistribution onto secondary resonances, dissipative redistribution onto the whole energy shell, and intramolecular mode inhomogeneity producing a frequency dispersion. The intramolecular couplings obey a hierarchy V k versus the number k of quanta exchanged, with an effective density of couplings ρ k . A model without arbitrary parameters settles that the parameter γ=πρ<V 2≳ can be shared between a dissipative part γ d ∼1–1.5 cm−1 (for E∼10 000–15 000 cm−1), corresponding to the orders k≥5, and a perturbative part γ p ∼2–15 cm−1, corresponding to the lowest orders k≂3 and 4, i.e., the strongest couplings cannot produce the dissipative relaxation. The same model explains that the QC threshold is at E∼4000 cm−1, as demonstrated by Raman experiments, with a half‐width γ d ∼0.21 cm−1, in excellent agreement with picosecond spectroscopy giving a T 1relaxation time of ∼11 ps. The parameter γ is also extracted from IRMPA cross sections. It is found from these data that γ(E) increases from 0.25 to 15 cm−1 when E increases from 4000 to 34000 cm−1. Finally, picosecond data, showing a spectacular enhancement of the number of photons absorbed for short pulses of duration τ L ∼30 ps, are fully explained by the previous QC description: Energy is mainly absorbed selectively in the ν3 mode, producing a dramatic enhancement of the Rabi width. Then, all known data on the SF6 QC can be incorporated in a unique framework, likely generalizable to other molecules.
Emission spectroscopy of nitrogen in a supersonic discharge. Evidence of excited neutral nitrogen clusters98(1993); http://dx.doi.org/10.1063/1.465073View Description Hide Description
Emission spectra indicating transient clusters in a supersonic nitrogen discharge are presented. Select spectral bands are attributed to emission from N2(C 3Π u ,v’=0,1)–N2(X 1Σ g +). The cluster transitions manifest as blue‐degraded, blue‐shifted sidebands of the second positive group due to emission from N2(C 3Π u ). Results and significance concerning these previously undiscovered nitrogen clusters are discussed.
98(1993); http://dx.doi.org/10.1063/1.464139View Description Hide Description
The transient species CH2S and HCS were studied by photoionizationmass spectrometry. They were prepared in situ from CH3SH by sequential hydrogen abstraction with fluorine atoms. CH2S was also prepared by pyrolysis of CH3SCl and CH3SSCH3. The photoion yield curve of CH2S displays an abrupt threshold, and is similar in overall shape to that of the homologue CH2O. The adiabatic ionization potential of CH2S is found to be 9.376±0.003 eV. Evidence has been found for nd and/or ns and npRydberg states converging to the first excited state of CH2S+. In addition, the HCS+ fragment from CH2S has been determined to appear at ≤11.533±0.021 eV at 0 K. In contrast to CH2S, the photoion yield curve of HCS+ from HCS displays a very broad Franck–Condon envelope, consistent with a transition from bent HCS to linear HCS+. A Poisson fit to the experimental Franck–Condon factors indicates that the adiabatic ionization potential of HCS is ≤7.499±0.005 eV, and perhaps as low as 7.412±0.007 eV. The fragment curves at m/e=46, 47, 48, and 49 from CH3SSCH3 have also been examined, and their relative shifts in energy determined. Together with measurements on CH2S and HCS, and the previously reported ΔH ○ f0 (CH2SH+)=211.5±2.0 kcal/mol (≤213.1±0.2 kcal/mol), this is sufficient to establish ΔH ○ f0 (CH2S)=28.3±2.0 kcal/mol (≤29.9±0.9 kcal/mol) and ΔH ○ f0 (HCS)=71.7±2.0 kcal/mol (≤73.3±1.0 kcal/mol), ≥69.7±2.0 kcal/mol). These values are in very good agreement with recent ab initio calculations. The implications for various bond energies within the CH n S system are also discussed.
Resonance effects of diabatic surface crossing within the torsional spectrum of 9‐(N‐carbazolyl) anthracene observed by supersonic jet fluorescence spectroscopy98(1993); http://dx.doi.org/10.1063/1.464140View Description Hide Description
Using the supersonic jet technique and laser‐induced fluorescence spectroscopy, the ground and excited statesurface of isolated 9‐(N‐carbazolyl) anthracene (C9A) is investigated. Ground and excited state torsional potentials of high accuracy are deduced from excitation and fluorescence spectra, considering characteristic patterns of Franck–Condon factors within the dispersed fluorescence.S 0 exhibits a very flat double minimum potential (equilibrium twist angle 77.5°, barrier 17 cm−1); the barrier for perpendicularity in S 1 is approximately 1050 cm−1 and the equilibrium angle is shifted towards coplanarity (64°). An unusual intensity profile of the long progression found in the fluorescence excitation spectrum is ascribed to a resonant nonradiative decay channel within the excited statesurface. State selective fluorescence decay rates vs excess vibrational energy confirm this resonant relaxation process. This uncommon observation leads to a model of diabatic surface crossing along the torsional coordinate where the crossing ‘‘dark’’ state is discussed as a predicted charge transfer state or a higher lying triplet state, mediating further electronic relaxation. Although extended intermolecular vibrational redistribution (IVR) is present in the fluorescence spectra from high vibrational levels, this process is of secondary importance for the resonant nonradiative relaxation.
Two‐photon absorption and second hyperpolarizability measurements in diphenylbutadiene by degenerate four‐wave mixing98(1993); http://dx.doi.org/10.1063/1.464141View Description Hide Description
We have investigated two‐photon absorption (TPA) in diphenylbutadiene at 532 nm using degenerate four‐wave mixing (DFWM) of nanosecond laser pulses. We present a theory which describes the development of double‐peaked phase conjugate pulses produced in DFWM due to TPA‐induced gratings superimposed on the usual electronic third‐order processes. Our analysis suggests a novel technique for measuring both the real and imaginary parts of the third‐order susceptibility of a molecular solution, hence leading to a determination of the TPA cross section σ2 and second hyperpolarizability γ of the solute molecules. Applying this experimental technique to diphenylbutadiene in chloroform, we obtain σ2=(40±8)×10−50 cm−4 s/photon–molecule and ‖γ‖=(420±80)×10−36 esu for this diphenyl polyene. We show that these values are in agreement with related measurements in diphenylbutadiene, and that they are consistent with a two‐photon resonance near 39 500 cm−1. The DFWM measurement technique we describe is very sensitive and should be applicable for measuring the two‐photon spectra of a variety of molecules.
State‐to‐state vibrational predissociation of H2–HF and D2–HF. Direct comparisons between theory and experiment98(1993); http://dx.doi.org/10.1063/1.464142View Description Hide Description
The optothermal detection method has been used to measure photofragment angular distributions resulting from the vibrational predissociation of both H2–HF and D2–HF. These angular distributions show resolvable structure which can be related to the final rotational state distributions, including the scalar intermolecular rotational correlations. H2–HF dissociates via a predominantly V–R process, while in D2–HF the open D2 vibrational channel is the main depository for the excess energy. The experimental results are compared directly with the recent time‐independent close‐coupling calculations of Clary [J. Chem. Phys. 96, 90 (1992)] and the time‐dependent golden rule calculations of Zhang, Zhang, and Bačić [J. Chem. Phys. 97, 927; 3149 (1992); Chem. Phys. Lett. 194, 313 (1992)]. The fact that these two systems have very different dynamics, while the potential‐energy surfaces are the same, is particularly useful in providing new insights into the nature of the coupling responsible for predissociation.
98(1993); http://dx.doi.org/10.1063/1.464143View Description Hide Description
A method of characterizing molecular spectra is proposed here. Hierarchical trees are generated from molecular spectra by following the branching of peaks as resolution is changed continuously. The trees are analyzed using several techniques developed in the classification and multivariate analysis literature. The information obtained from these techniques includes (1) dimensionality of geometrical representations of the trees; (2) clustering statistics; and (3) number of distinct time scales inherent in the spectra. Several model spectra are studied to establish the utility of the hierarchical analysis and a portion of a vibronic spectrum of NO2 is also investigated.
The core‐penetrating Rydberg series of the CaF molecule: At the borderline between valence and Rydberg states98(1993); http://dx.doi.org/10.1063/1.464144View Description Hide Description
This paper reports the results of a pulsed optical–optical double resonance study of three new low energy Rydberg states and the previously known but not rotationally analyzedF 2Π state [C. A. Fowler, Phys. Rev. 59, 645 (1941)] of the CaF molecule. The v=0 levels of these four electronic states are in the energy region 36 000–39 800 cm−1 (n*≊3‐4). Molecular constants (A,p,q) or γ, and B were measured for all of the states. The F’ 2Σ+ state at 36 125 cm−1 (n*=3.177) is particularly important because it is the lowest and previously unobserved member of the ’p’ 2Σ+Rydberg series. The assignment of the F’ state as the lowest member of the ’p’ 2Σ+ series corrects a mistaken assignment of the C’ 2Σ+ state as the lowest member of the ’p’ 2Σ+ series [J. E. Murphy, J. M. Berg, A. J. Merer, N. A. Harris, and R. W. Field, Phys. Rev. Lett. 65, 1861 (1990)].
Generation of an exact three‐dimensional quadrupole electric field and superposition of a homogeneous electric field within a common closed boundary with application to mass spectrometry98(1993); http://dx.doi.org/10.1063/1.464145View Description Hide Description
The exact three‐dimensional rotationally symmetric quadrupole field can be generated by closed boundaries with continuously varied potential, especially with linearly variable potential, in the ideal case by simple cone‐shaped boundaries with linearly variable potential. An example for the application of the field is the storage of charged particles inside the closed boundaries. Within the same cone‐shaped boundaries, a homogeneous ideal field in the direction of the symmetry axis can be generated and superimposed without interaction with the quadrupole field. This field can be employed for excitation of the kinetic energy of the stored charged particles. Potential distribution of both fields within one cone‐shaped boundary with linearly variable potential are simulated separately. A new electrode structure for ion trap is suggested. The signals induced by the coherently moving ions on the electrodes can be analyzed by Fourier transformation.
98(1993); http://dx.doi.org/10.1063/1.464146View Description Hide Description
Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600±200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time‐of‐flight mass spectrometry. As the average cluster size increases, the x‐ray absorptionspectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2p 3/2→4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x‐ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2p 3/2→4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2pEXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2pEXAFS and 4s exciton signals.
98(1993); http://dx.doi.org/10.1063/1.464147View Description Hide Description
We present near steady state photoinduced absorption spectra of two forms of polyaniline. Both the emeraldine base (nondegenerate ground state) and pernigraniline (degenerate ground state) exhibit long lived photoinduced nonlinear excitations with transient absorptions within the π→π* gap. In the case of the emeraldine base, two well‐resolved photoinduced absorption bands are observed with maxima at 0.9 and 1.47 eV, accompanied by a bleaching of the 2 eV absorption edge. The different relaxation rates of the two bands indicate that they arise from two independent photoinduced nonlinear excitations. The results are discussed in terms of intrachain and interchain excitations. The possibility of deep traps on defect sites in pernigraniline is suggested.
98(1993); http://dx.doi.org/10.1063/1.464148View Description Hide Description
The Doppler‐free high resolution laser spectroscopy of Cs2 D 1Σ u +–X 1Σ g + transition is extended up to v’=65. By comparing the spectral linewidth and the time‐resolved fluorescence intensity, the line broadening observed for transitions to the D 1Σ u +(v’=63,J’≤70) levels is identified as the lifetime broadening originating from the predissociation. Line splittings are observed for the D 1Σ u +(v’=46,J’≥95)–X 1Σ g +(v‘= 1,J‘) transitions and are identified as the hyperfine splitting due to a magnetic dipole interaction between nuclear spin and electron. The hyperfine splitting is attributed to mixing of the (2) 3Π u state, whose wave function changes from Hund’s case (a) to case (b) at large J. The dependence of the electric dipole transition moment on the internuclear distance for the D 1Σ u +–X 1Σ g + transition is determined by comparing the observed and calculated line intensities of the dispersed fluorescence.
98(1993); http://dx.doi.org/10.1063/1.464149View Description Hide Description
The lowest Rydberg stateE(3Σ+) of the HgNe van der Waals complex has been investigated by optical–optical double resonance (OODR) spectroscopy using two intermediate electronic states of A 30+ and B 31. The E–B band exhibits an oscillatory free–bound continuum, which reflects a Franck–Condon projection of the wave function of the B state onto the repulsive part of the E state potential. In the E–A band, two relatively sharp peaks are observed together with a free–bound continuum showing an asymmetric interference structure. The observed intensity patterns of the E–B and E–A bands are interpreted by a potential barrier characteristic of the E state potential, which traps two quasibound vibrational states in the inner well. Based on (i) the observed Franck–Condon pattern of the free–bound transitions; (ii) the transition’s wave number of the bound–bound transitions in the E–A band; and (iii) the rotational constants of the quasibound (v=0 and v=1) levels in the E state, the interatomic potential of the E state is derived by a numerical simulation. The height of the potential barrier measured from the dissociation limit of Hg (7 3 S 1)+Ne is determined to be 153 cm−1 and the interatomic distance at the top of the barrier to be located at 3.9(1) Å.
High resolution laser spectroscopy of asymmetrically deuterated cyclopentadienyl radicals: A study of vibronic degeneracy resolution and Jahn–Teller distortion98(1993); http://dx.doi.org/10.1063/1.464150View Description Hide Description
The rotationally resolved, laser induced fluorescence, excitation spectra of the partially deuterated cyclopentadienyl radicals, C5H4D and C5HD4, have been observed at low temperature in a supersonic free jet expansion. The observed electronic transition in the uv region corresponds to the Ã 2 A ‘ 2■X̃ 2 E ‘ 1 transition in the symmetric cyclopentadienyl isotopomers with D 5h symmetry. In the reduced C 2v symmetry of the C5HD4 and C5H4D isotopomers, this electronic transition splits into two distinct vibronic bands, separated by about 9 cm−1, which arise from the two vibronic components X̃1 and X̃2 into which the X̃ state is resolved when the symmetry is lowered. In C5H4D the ground X̃1 state has 2 A 2 symmetry and a permanently distorted, elongated allyl‐like structure while the low‐lying X̃2 state has 2 B 2 symmetry and a compressed dienelike structure. The symmetries of the energy levels and the distortions are reversed for the C5HD4 species. A detailed theoretical model is developed to describe the splitting and the rotational structure of the X̃1 and X̃2 states. Application of this model yields a precise value for the alternation of the C–C bond lengths in the distorted structures. This value is shown to be the same as that describing the dynamically pseudorotating, Jahn–Teller distorted structures in the D 5h isotopomers. Additionally, the theory explains anomalous values of the observed inertial defects and relates them to physically meaningful quantities for all the isotopomers in both their X̃1 and X̃2 states. Our results are compared to previous experimental work and ab initio calculations on the cyclopentadienyl radical.
98(1993); http://dx.doi.org/10.1063/1.464151View Description Hide Description
Silver dimers formed in a seeded supersonic argon beam are examined with two laser spectroscopic methods. Excitation fluorescence spectra of the A‐X system excited with a narrow band cw dye laser yield accurate constants of the X 1Σ g + and the A 1Σ u + state. From isotope selective resonant two‐photon ionization spectra of the B‐X and the E‐X system excited by a pulsed dye laser and monitored with a time‐of‐flight mass spectrometer, improved vibrational constants and rotational constants of the B 1Π u and the E 1Π u state are derived. Rotational constants of the C and the D state could be determined from the spectral separations between bandhead and band origin in partly rotationally resolved bands. Autoionizing Rydberg states of Ag2 are stepwise excited with two pulsed dye lasers in two resonant steps. From the convergence limits of different Rydberg series converging towards different vibrational levels v + in the X 2Σ g +ground state of Ag2 + the rotational constants of the ion ground state and the adiabatic ionization potential IP(Ag2 +)=61 747±4 cm−1 could be accurately determined.