Volume 85, Issue 12, 15 December 1986
Index of content:
85(1986); http://dx.doi.org/10.1063/1.451847View Description Hide Description
Luminescence lifetimes for two gas europium chelate complexes have been measured as a function of gas temperature. Following ligand excitation the rate constant for Eu(+3) 5 D 0→7 F 2 in both molecules shows an Arrhenius temperature dependence. An activation energy of 5100 cm− 1 is obtained for europium (3+) tris‐1, 1, 1, 2, 2, 3, 3‐heptafluoro‐7, 7‐dimethyl‐4, 6,‐octanedione, Eu(fod)3, and 4100 cm− 1 is obtained for the activation energy of europium (3+) tris‐2, 2, 6, 6,‐tetramethyl‐3, 5‐heptanedione, Eu(thd)3. The quantum yield for production of molecules in the luminescing Eu(+3) 5 D 0 state is found to be independent of temperature when either Eu(thd)3 or Eu(fod)3 is excited in the ultraviolet absorption bands of the ligands.
The rotational spectrum and molecular geometry of an antihydrogen‐bonded dimer of sulfur dioxide and hydrogen cyanide85(1986); http://dx.doi.org/10.1063/1.451420View Description Hide Description
The ground‐state rotational spectra of the four isotopic species (3 2SO2,HC1 4N), (3 2SO2,DC1 4N), (3 2SO2,HC1 5N), and (3 4SO2,HC1 4N) of a weakly bound dimer of sulfur dioxide and hydrogen cyanide have been observed by means of pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The 1 4N‐nuclear quadrupole coupling constants determined for the isotopomer (3 2SO2,HC1 4N) of this asymmetric rotor molecule are χ a a =−2.876(3) MHz and χ b b =2.025(4) MHz while the rotational and centrifugal distortion constants are (A 0−Δ K ) =8633.849(1) MHz, B 0=1848.890(2) MHz, C 0=1615.863(2) MHz, Δ J =9.56(1) kHz, Δ J K =114.84(7) kHz, δ J =0.838(6) kHz, and δ K =89(1) kHz. The spectroscopic constants have been interpreted in terms of a nonplanar, antihydrogen‐bonded geometry of C s symmetry with S, H, C, and N nuclei lying in the symmetry plane. The HCN molecule is roughly perpendicular to the plane of the SO2 molecule with N lying between the SO2 plane and the H atom. The distance between the SO2 and HCN centers of mass is r c.m. =3.6542(1) Å. The angle between the C 2 axis of the SO2 subunit and the NCH axis is 86° and the two axes intersect at a distance of 1.38 Å above the S nucleus.
85(1986); http://dx.doi.org/10.1063/1.451421View Description Hide Description
A novel and general concept of restricting coherence transfer in nuclear spin systems is described. It opens new possibilities for editing one‐ and two‐dimensional NMR spectra. For example, the widely applied two‐dimensional correlation experiment COSY can be modified such as to restrict coherence transfer to take place exclusively between connected transitions in the energy level diagram. Such two‐dimensional spectra possess ideal features for assignment of complex scalar coupling networks and for computer assisted analysis. Experimental 1H spectra of a cyclic decapeptide are presented. Other applications of the general filtering concept are briefly discussed.
Adiabatic corrections to the potential energy curves of the X 1∑+ state of the isotopic lithium hydrides85(1986); http://dx.doi.org/10.1063/1.451422View Description Hide Description
From isotopic spectroscopic data, the internuclear distance dependence of the adiabatic corrections to the potential energy curve has been determined for the ∑ state of a diatomic molecule. Starting from an analytic inversion procedure previously described, the adiabatic corrections can be found in a straightforward way, provided that they can be considered as perturbing terms of the vibration–rotation wave equation. Application to the case of the X 1∑+ state of the lithium hydrides 6LiH, 7LiH, 6LiD, and 7LiD is carried out. The adiabatic corrections ΔU H(R) and ΔU Li(R) are obtained and compared with recent results.
85(1986); http://dx.doi.org/10.1063/1.451423View Description Hide Description
The elastic properties of N‐isopropylcarbazole (NIPC), a pyroelectric molecular crystal, are investigated by Brillouin scattering. The full elastic constant tensor is determined at 295 K and the temperature dependences of the elastic constants are given. The major experimental finding reported is the anomaly of the LA a‐axis mode governed by the c 1 1elastic constant. This mode exhibits a pronounced downward bending on both sides of the nonferroic, first order phase transition at ∼137 K. Symmetry allowed linear‐quadratic and biquadratic couplings between the Brillouin zone boundary one‐dimensional order parameter and the zone center strain introduced into the Landau free energy do not account for the observed anomaly. The transition is characterized in terms of a strong dispersion of the c 1 1elastic constant and a large dynamical critical behavior. By combining the c 1 1elastic constant data and the Brillouin scattering LA a‐axis mode half‐width data through a Landau–Khalatnikov process, one can extract a relaxation time satisfying a mean‐field dependence characteristic of critical slowing down of the order parameter.
A comparative study of the fluorescence lifetimes of 9‐cyanoanthracene in a bulb and supersonic free jet85(1986); http://dx.doi.org/10.1063/1.451424View Description Hide Description
The fluorescence decays of 9‐cyanoanthracene (9CNA) in a supersonic free jet have been studied for a large number of vibrational bands including the electronic band origin. The fluorescence lifetime τ f varied from vibrational level to level but decreased from 28.0±0.2 at the 0–0 band origin to 4.2±0.1 ns at the excess energy of 2045 cm− 1. In a bulb, however, τ f of 9CNA was no more than 3.2 ns and independent of both temperature (433–553 K) and excess energy (∼4800 cm− 1). We attempted to interpret the very short and temperature invariant τ f of 9CNA in the bulb in terms of the vibrational level specific τ f found in the supersonic jet. No simple way to rationalize the findings in the bulb was found but the results suggested an important role of optically nonactive vibrational modes in determining the radiationless transition. The nonexponential decay with the lifetimes of 3.9±0.4 and 27.3±0.2 ns found for the partially cooled state of 9CNA, which was created by seeding 9CNA in helium at the pressure of 200 Torr, can be taken as the evidence supporting this view.
Determination of population and alignment of the ground state using two‐photon nonresonant excitation85(1986); http://dx.doi.org/10.1063/1.451374View Description Hide Description
A method is presented for determining the population A (0) 0, the quadrupole alignment factors A (2) 0, A (2) 1, A (2) 2, and the hexadecapole alignment factors A (4) 0, A (4) 1, A (4) 2, A (4) 3, A (4) 4 for a (v,J) ground state distribution of a diatomic molecule probed by linearly polarized two‐photon nonresonant excitation. General expressions are developed for the O, P, Q, R, and S branch transitions as a function of the rotational quantum number J. This treatment assumes that the resonant state reached by the two‐photon transition is subsequently detected independent of its alignment. This can be achieved by 2+nmultiphoton ionization in which the ionization steps are saturated, or by 2+1 laser induced fluorescence in which the fluorescence is collected independent of its polarization and spatial anisotropy. To extract the population and the eight alignment parameters the line intensities must be measured for several polarization settings of the laser beam. However, when the ground state distribution has cylindrical symmetry, only two alignment parameters are nonvanishing, A (2) 0 and A (4) 0, and they can be determined at a single polarization setting by comparing the line intensities of the different branches.
85(1986); http://dx.doi.org/10.1063/1.451375View Description Hide Description
The ground‐state rotational spectra of five isotopic species of a weakly bound dimer formed between methyl cyanide and acetylene have been investigated by pulsed‐nozzle, Fourier‐transform microwave spectroscopy. The rotational constantB 0, the centrifugal distortion constants D J and D J K , and the 1 4N–nuclear quadrupole coupling constant χ(1 4N) have been determined for the symmetric top species CH3C1 4N⋅⋅⋅HCCH, CH3C1 4N⋅⋅⋅DCCH, CH3C1 4N⋅⋅⋅HCCD, CH3C1 4N⋅⋅⋅DCCD, and CH3C1 5N⋅⋅⋅HCCH. The values for the parent isotopic species are B 0=977.4659(1) MHz, D J =0.718(1) kHz, D J K =139.36(7) kHz, and χ(1 4N)=−3.95(6) MHz. The nature of the observed spectra and the magnitudes of the B 0 values and D J values are interpreted in terms of a hydrogen‐bonded geometry having C 3v symmetry, with the nuclei in the order H3CCN⋅⋅⋅HCCH, with r(N⋅⋅⋅C)=3.425(1) Å and with the hydrogen bond stretching force constant k σ=4.7(1) Nm− 1.
85(1986); http://dx.doi.org/10.1063/1.451376View Description Hide Description
The optothermal laser–molecular beam method has been used to measure the infrared spectrum of Ar–HF [10°0←00°0]. The results show that the vibrational predissociation lifetime of this complex is greater than the flight time of the molecules from the laser crossing region to the bolometerdetector. This gives a lower limit on the lifetime of 3×10− 4 s! The upper vibrational statedipole moment has also been obtained for the complex (μ1=1.495 D) by carrying out infrared stark spectroscopy. This corresponds to a 12% increase in the dipole moment upon vibrational excitation. This change can be related to a stiffening of the van der Waals bond, and hence a reduction in the amplitude of the bending motion, in the vibrationally excited state.
85(1986); http://dx.doi.org/10.1063/1.451377View Description Hide Description
The chloronium ion (ClH+ 2) has been detected in the gas phase by infrared diode laser spectroscopy with magnetic field modulation. The ion was generated by a hollow‐cathode discharge in an H2, He, and HCl mixture. One hundred and forty eight lines were observed between 1040–1330 cm− 1, of which 99 and 25 were assigned to the ν2 fundamental bands of 3 5ClH+ 2 and 3 7ClH+ 2, respectively. The observed spectra were analyzed by using Watson’s A‐reduced Hamiltonian to determine the band origins [ν2(3 5ClH+ 2) =1184.1256(2) cm− 1, ν2(3 7ClH+ 2) =1183.2165(13) cm− 1] and rotational and centrifugal distortion constants. The r 0 structure was calculated for 3 5ClH+ 2 to be r 0(H–Cl)=1.3135(87) Å and θ0(HClH)=94.3(12)° with standard errors in parentheses.
Sensitive quantum state selective detection of H2O and D2O by (2+1)‐resonance enhanced multiphoton ionization85(1986); http://dx.doi.org/10.1063/1.451845View Description Hide Description
The first observation of (2+1)‐REMPI of H2O and D2O is reported. With the use of a high power tunable excimer laser radiating at 248 nm, the H2O and D2O molecules are ionized after resonant two‐photon absorption into the predissociated C̃ 1 B 1 state. The clearly observable peaks in the (2+1)‐REMPI spectra are all identified and can be used for sensitive state selective detection. Parent molecular fluorescence excitation spectra (C̃ 1 B 1→Ã 1 B 1) were remeasured over an increased spectral range, and are remarkably the same as the (2+1)‐REMPI spectra. Furthermore the OH/OD (A 2Σ+, v’=0→X 2Π, v‘=0) photofragment fluorescence excitation spectra were measured, and these spectra do not show any nonresonant background as stated before. Additional fluorescence bands starting from the vibrationally excited A 2Σ+, v’=1 state were observed. Simulation of the observed REMPI and fluorescence excitation spectra yields the branching ratios for the predissociation,ionization, and fluorescence processes.
85(1986); http://dx.doi.org/10.1063/1.451378View Description Hide Description
The rf pulse response of a system of I=3/2 nuclei which experience identical first‐order quadrupole splitting is investigated using the density matrix method. A general expression is derived for the time evolution of the in‐phase magnetization following a rectangular pulse applied at the center line frequency. The maximum amplitude of the magnetization associated with the center line, and the length of pulse that gives rise to it, are calculated as a function of the ratio of the quadrupole splitting to the amplitude of the rf field. Data obtained for 2 3Na in a single crystal of NaNO3 are in fairly good agreement with the theoretical form. The theory has been extended to the case where the nuclei are situated at equivalent sites in a polycrystalline sample. The length of pulse that maximizes the amplitude of the FID of 2 3Na in a powder sample of NaNO3 has been measured as a function of the amplitude of the rf field. The quadrupole coupling constant, found by fitting the data to the theoretical form, is in very good agreement with the accepted value. It is proposed that this method can be widely used to measure the quadrupole coupling constants of I=3/2 nuclei.
85(1986); http://dx.doi.org/10.1063/1.451379View Description Hide Description
The 584 Å photoelectron spectra of supersonic molecular beams of H2O and D2O have been obtained with improved resolution. The spectroscopic constants of the X̃ 2 B 1 and Ã 2 A 1 state ions, including ω0 1, x 0 11, ω0 2, x 0 22, and x 0 12, are reported. For the first two electronic states of the ion, precise line splittings were evaluated with a least squares fitting procedure, employing sums of empirical instrument response functions and a linear background. A simulation of the vibrational manifolds of the B̃ 2 B 2 state ions with combination progressions in the symmetry‐allowed modes ν1 and ν2 failed to reproduce the diffuse photoelectron bands observed for both H2O and D2O. Autocorrelation functions were calculated from the photoelectron bands of all three electronic states. The B̃ 2 B 2 state correlation functions exhibit ultrafast decay, occurring on a 10− 1 4 s time scale. The ν2 motion appears to define the decay in the correlation function. This behavior supports a previously proposed B̃ 2 B 2–Ã 2 A 1 curve‐crossing model for the nonradiative relaxation of the B̃ 2 B 2 state ions.
85(1986); http://dx.doi.org/10.1063/1.451380View Description Hide Description
Some rotational components in the v 9=2 vibrational state of propyne have been determined in the frequency range 17–72 GHz. Molecular constants for this vibrationally excited state have been determined from more than 11 observed rotational transitions. Experimentally measured frequencies are presented and compared with those calculated using the results of basic perturbation theory. A constant set was obtained for the v 9=1 and v 9=2 vibrationally excited levels using the experimental data obtained for the ground and these two vibrational levels. Agreement was found to be quite good for all except one component which may be perturbed by combination bands.
Dispersed fluorescence of jet‐cooled tryptophan: Excited state conformers and intramolecular exciplex formation85(1986); http://dx.doi.org/10.1063/1.451381View Description Hide Description
The dispersed fluorescence of the amino acid tryptophan has been measured in the environment of a cold, supersonic free jet. Analysis of the region of the spectrum near the electronic origin indicates that the electronic excitation spectrum contains features which arise from various ground state conformers of tryptophan, confirming our previous assignment of these features. Under the conditions of our experiment the conformers do not interconvert in the excited state during the fluorescence liftime. Analysis of the dispersed emission spectrum of one conformer reveals broad red‐shifted fluorescence which exists even when the electronic origin transition is excited. This broad red‐shifted fluorescence is produced by the formation of an intramolecular exciplex involving excited stateproton transfer to form a zwitterion. Molecules which do not have the ability to form a zwitterion do not exhibit this behavior, and deuterated trytophan shows broad fluorescence in an amount consistent with a slower proton transfer rate. The significance of these results for understanding the excited state photophysics of tryptophan in solution is discussed.
85(1986); http://dx.doi.org/10.1063/1.451382View Description Hide Description
We have used one‐color, resonance enhanced multiphoton ionization in the near‐UV to create selectively the first excited terms of the transition metalions Fe+, Ti+, and V+ in the gas phase. The term and level distributions of the resulting photoions are measured using time‐of‐flight photoelectron spectroscopy. We generally find better electron configuration and term selectivity for two‐photon ionization via 4p resonant Rydberg levels than was found in previous work on three‐photon ionization of 5sRydberg levels. In certain Fe cases, a single ion total angular momentum level dominates the photoelectron spectrum. The simple picture of the 4pRydberg levels as consisting of an ion core of well defined L c –S c and J c weakly coupled to a Rydberg electron is remarkably useful. Deviations from this simple picture are discussed in terms of configuration interaction of resonant states and autoionizing structure in the continuum. The results will find application in studies of state‐selected metal ion chemistry.
85(1986); http://dx.doi.org/10.1063/1.451383View Description Hide Description
Precise isosbestic points occur in the Raman OH‐stretching spectra from liquid water between 3 and 85 °C if cell alignment is accomplished with Newton’s rings. Isosbestic frequencies measured for the orientations X(Y,X+Z)Y=6β2, X(Z X)Y=3β2, X(Y+Z,X+Z)Y=45α2+13β2, X(Z,X+Z)Y=45α2+7β2, and X(Z Z)Y=45α2+4β2 are 3524, 3522 (note β2 agreement), 3468, 3425, and 3403 cm− 1, respectively. Isosbestic points from two different measurements calculated by the relations, X(Z Z)Y‐(4/3)X(Z X)Y and X(Z,X+Z)Y‐(7/6)X(Y,X+Z)Y agree exactly for 45α2, 3370 cm− 1. (α and β2 correspond to the mean polarizability and square of the anisotropy.) The pure α2 isosbestic frequency, 3370 cm− 1, coincides with the peak of the highest frequency hydrogen‐bonded (HB) Gaussian OH‐stretching component. The pure β2 isosbestic point, 3522–3524 cm− 1, coincides with the peak of the nonhydrogen‐bonded (NHB) Gaussian OH‐stretching component, next above in frequency. The α2 and β2 isosbestic points are thus thought to provide an experimental distinction between, and a clear definition of, the HB and NHB OH‐oscillator classes for water. Moreover, the various OH‐stretching combinations of α2 and β2 simply provide different measures of the
HB→NHB equilibrium—no special information concerning the temperature dependence of this equilibrium results from use of any one linear polarizability combination over any other, including pure α2 or pure β2. The present results agree with mercury‐excited data [Walrafen, J. Chem. Phys. 4 7, 114 (1967)] for X(Y+Z,X+Z)Y and with the corrected α2 data of d’Arrigo e t a l. [J. Chem. Phys. 7 5, 4264 (1981)]. Furthermore, the new data are in accord with the spectroscopic mixture model, but the continuum model conflicts with the observation of exact points. The isosbestic frequencies are also found to be strongly nonlinear in the amount of α2 or β2 involved in the spectra.
85(1986); http://dx.doi.org/10.1063/1.451384View Description Hide Description
Low frequency Δν̄=0–350 cm− 1, Raman intensity data were obtained from liquid water between 3.5 and 89.3 °C using holographic grating double and triple monochromators. The spectra were Bose–Einstein (BE) corrected, I/(1+n), and the total integrated (absolute) contour intensities were treated by an elaboration of the Young–Westerdahl (YW) thermodynamic method, assuming conservation of hydrogen‐bonded (HB) and nonhydrogen‐bonded (NHB=bent and/or stretched, O–H O) nearest‐neighbor O–O pairs. A ΔH ° 1 value of 2.6±0.1 kcal/mol O–H ⋅⋅⋅ O or 5.2±0.2 kcal/mol H2O (11 kJ/mol O–H ⋅⋅⋅ O, or 22 kJ/mol H2O) resulted for the HB→NHB process. This intermolecular value agrees quantitatively with Raman and infrared ΔH° values from the one‐ and two‐phonon OH‐stretching regions, and from molecular dynamics, depolarized light scattering,neutron scattering, and ultrasonic absorption, thus indicating a common process. A population involving partial covalency of, i.e., charge transfer into, the H ⋅⋅⋅ O units of linear and/or weakly bent hydrogen bonds, O–H ⋅⋅⋅ O; is transformed into a second high energy population involving bent, e.g., 150° or less, and/or stretched, e.g., 3.2 Å, but otherwise strongly cohesive O–H O interactions. All difference spectra from the fundamental OH‐stretching contours cross at the X(Z,X+Z)Y isobestic frequency of 3425 cm− 1. Also, total integrated Raman intensity decreases occurring below 3425 cm− 1 with temperature rise were found to be proportional to the total integrated intensity increases above 3425 cm− 1, indicating conservation among the HB and NHB OH‐stretching classes. From the enthalpy of vaporization of water at 0 °C, and the ΔH ° 1 of 2.6 kcal/mol O–H ⋅⋅⋅ O, the additional
enthalpy, ΔH ° 2, needed for the complete separation of the NHB O–O nearest neighbors is ∼3.2 kcal/mol O–H ⋅⋅⋅ O or ∼6.4 kcal/mol H2O (13 kJ/mol O–H ⋅⋅⋅ O or 27 kJ/mol H2O). The NHB O–O nearest neighbors are held by forces other than those involving H ⋅⋅⋅ O partial covalency, i.e., electrostatic (multipole), induction, and dispersion forces. The NHB O–O pairs do not appear to produce significant intermolecular Raman intensity because they lack H ⋅⋅⋅O bond polarizability, but the corresponding NHB OH oscillators do contribute weakened Raman intensity above 3425 cm− 1. An ideal solution thermodynamic treatment employing ΔH ° 1 =2.6 kcal/mol O–H ⋅⋅⋅ O, the HB mole fraction, and the vapor heat capacity, yielded a very satisfactory specific heat value of 1.1 cal deg− 1 g− 1 H2O at 0 °C. The NHB mole fraction, f u , from the YW treatment is negligibly small, 0.06 or less, for t<−50 °C. However, f u increases to 0.16 at 0 °C, and f u ≊1 at 1437 °C, where recent shock‐wave Raman measurements indicate loss of all partially covalent, charge transferhydrogen bonding.
Doppler‐free saturation spectroscopy of polyatomic molecules: Photochemical hole burning of gas phase s‐tetrazine85(1986); http://dx.doi.org/10.1063/1.451385View Description Hide Description
Doppler‐free spectra of two vibronic bands in the Ã (1 B 3 u )←X̃ (1 A g ) transition of the photochemically instable s‐tetrazine (H2C2N4) are presented. For the first time saturation spectroscopy is successfully applied to a large polyatomic molecule. Photochemical decomposition of s‐tetrazine molecules takes place after excitation and prevents the molecule from returning to the ground state by radiation and nonradiative processes. This represents a particular type of hole burning in the ground state velocity distribution. The elimination of the inhomogeneous Doppler broadening enables us to determine collisionless homogeneous linewidths. For the 0–0 band a value of γ=190 MHz is found whereas the 16a 1 1 band at higher excess energy (ν’ 16a =256 cm− 1) shows a sharper linewidth of γ=140 MHz. No rotational dependence of the linewidth is found in the 0–0 band up to J levels higher than 70. It is argued that internal conversion to the X̃ (1 A g ) state is the process responsible for the observed linewidths and that dissociation takes place on the electronic ground statepotential surface.
85(1986); http://dx.doi.org/10.1063/1.451386View Description Hide Description
FTIRspectra of the matrix‐isolated monomeric and complexed H2S in solid O2 at 13 K have been obtained at various matrix: solute ratios (M/S) in order to identify the ν3 absorption of the monomer. The ν1 absorption of the monomer was too weak to be seen at high dilutions, M/S∼4000 and 8000. The previous assignments of the monomer ν1 absorption in Ar and N2 are questioned. The satellite peaks appearing at low values of (M/S) are assigned to (H2S)2. The observed H‐bonding dimer frequency, −50 cm− 1 from v 3, is comparable to the value calculated using second‐order Mo/ller‐Plesset perturbation theory. Frequencies calculated at the Hartree–Fock level for H2S dimer are essentially unchanged from those in the monomer.