Volume 88, Issue 4, 15 February 1988
Index of content:

Rotationally resolved double resonance spectra of NO Rydberg states near the first ionization limit
View Description Hide DescriptionOptical–optical double resonance multiphoton ionizationspectroscopy is used to study several Rydberg states of nitric oxide with the A(3sσ) ^{2}Σ^{+}, v=1 state as an intermediate level. These v=1 Rydberg states include the 8s, 9s, 7f, 8f, and 8p, but no d states are observed. The states are rotationally resolved due to the capability of optical–optical double resonance experiments to select particular rotational levels of the intermediate state. A rotationally analysis of the data for the 8s and 9s states yields values for the rotational constant, the centrifugal distortion constant, and the term energy of the lowest rotational level for each state. Analysis of the data for the 7f and 8f states yields values for the ion core quadrupole moment, polarizability and rotational constant, and a correction for core penetration. These constants are obtained by a generalized least‐squares fit to a long‐range interaction model for electrostatic forces between the ion core and the Rydberg electron. The 8p, v=1 state is perturbed, probably by an interaction with the 5f, v=2 state. One example of quantum interference between the 5f level with v=2, N=4, and L=−3, and the 8p ^{2}Σ^{+}, v=1, N=4 level has been analyzed.

Millimeter and submillimeter wave spectroscopy of protonated and deuterated nitrous oxide
View Description Hide DescriptionThe a‐ and b‐type rotational spectra of the protonated and deuterated nitrous oxide have been investigated in the millimeter and submillimeter frequency range. The molecular ions were produced in a magnetically confined glow discharge. The comparison between the accurate molecular constants deduced from the analysis of the spectra and different sets deduced from a b i n i t i o calculations gives structural information on the observed species: the experimental spectra belong to the O‐protonated and ‐deuterated nitrous oxide.

Laser spectroscopy of calcium and strontium monoazide free radicals
View Description Hide DescriptionWe have synthesized the gas‐phase metal azides CaN_{3} and SrN_{3}. These ionic metal monoazides were found to have linear geometries. The positions of the Ã^{2}Π–X̃ ^{2}Σ^{+} and B̃ ^{2}Σ^{+}–X̃ ^{2}Σ^{+} electronic transitions were determined as well as several vibrational frequencies. The 0–0 band of the Ã ^{2}Π–X̃ ^{2}Σ^{+} system of SrN_{3} has been rotationally analyzed by laser excitation spectroscopy yielding a Sr–N bond length of 2.26 Å.

High‐resolution laser spectroscopy of strontium isocyanate, SrNCO
View Description Hide DescriptionThe 0–0 band of the Ã ^{2}Π–X̃ ^{2}Σ^{+} transition of the linear free radical strontium monoisocyanate, SrNCO, was recorded at high resolution. By comparing the molecular constants of SrNCO with related molecules, the NCO^{−} ligand was found to be nitrogen bonding to the strontium atom. The spectrum is extremely dense because of the small rotational constant,B‘=0.0426 cm^{−} ^{1}, and overlapping sequence structure. The Sr–N bond length was estimated to be 2.26 Å in the ground X̃ ^{2}Σ^{+} state.

Infrared flash kinetic spectroscopy of HCO
View Description Hide DescriptionThe high resolution infrared spectrum of the CH stretching fundamental of the formyl radical (HCO) has been observed by means of infrared kinetic spectroscopy using 308 nm (XeCl) excimer laser flash photolysis of formaldehyde or acetaldehyde followed by diode or difference frequency laser probing of the transient absorption. The high resolution spectra obtained were assigned and fitted with rotational, spin–rotational, and centrifugal distortion constants. The ν_{1} band origin is 2434.48 cm^{−} ^{1}. New ground‐state constants are reported from a least‐squares fit combining this ν_{1} data with previous microwave and FIR LMR measurements.

ESR spectrum of I^{1} ^{7}O_{2}: Asymmetric hyperfine tensor of ^{1} ^{7}O
View Description Hide DescriptionESRspectra of I ^{1} ^{7}O_{2} embedded in solid KC10_{4} exhibit a complicated ^{1} ^{7}O hyperfine structure, which allows an accurate determination of the hyperfine and quadrupoletensorsA(^{1} ^{7}O) and Q(^{1} ^{7}O). A(^{1} ^{7}O) is strongly anisotropic in the molecular plane and contains an asymmetric component. These unexpected features are accounted for in terms of second‐order contributions to A(^{1} ^{7}O) arising from cross terms of the spin–orbit coupling and the orbital and spin dipole hyperfine interaction. The second‐order terms also contribute substantially to the hyperfinetensor of the iodine nucleus.

A relaxation time study of SF_{5} reorientation in solid tetrakis (pentafluorosulfanyl) hydrazine, (SF_{5})_{2}NN(SF_{5})_{2}
View Description Hide DescriptionAn experimental and theoretical study of the ^{1} ^{9}F relaxation in solid tetrakis (pentafluorosulfanyl) hydrazine due to the reorientation of the SF_{5} groups has been undertaken. T _{1} and T _{1ρ} have been measured from 138 to 292 K. T _{1} is very long, reaching a maximum of 1140 s at the lowest temperature for one sample, and is determined primarily by paramagnetic impurities. T _{1ρ} appears to be influenced by paramagnetic impurities below 227 K and determined by the reorientation of the SF_{5} groups above 227 K. Expressions for T _{1} and T _{1ρ} have been derived for two different reorientational models. In the first model the SF_{5} group is assumed to undergo 90° jumps about the C _{4} axis; whereas, in the second model the SF_{5} group is assumed to undergo rotational diffusion about the C _{4} axis. The T _{1ρ} measurements above 227 K have been fit to the derived expressions to obtain the correlation time for the reorientation.

^{2}H‐NMR study of the glass transition in supercooled ortho‐terphenyl
View Description Hide DescriptionThe glass forming molecular liquid ortho‐terphenyl has been investigated by ^{2}H‐NMR techniques providing spin‐relaxation times (T _{1}, T _{2}), and spin‐alignment data which yield information on the time scale and geometry of ultra‐slow molecular reorientation. The main results are as follows: The primary glass transition (α process) is characterized by rotational molecular jumps with a jump size distribution weighted in favor of large jump angles, and by a distribution of correlation times. In addition intramolecular flip–flop jumps of the lateral phenyl rings are found which do not take part in the α process. Apart from this (secondary) intramolecular dynamics no residual small angle reorientation persists below T _{ g } on the time scale (10^{−} ^{4} to 10^{2} s) of the spin‐alignment experiment.

Half‐collision studies of the Hg–NH_{3} excimer
View Description Hide DescriptionThe Hg–NH_{3} complex has been studied by forming the complex in a supersonic jet and probing the bound‐to‐bound transitions to the two excited electronic states correlated to Hg(6 ^{3} P _{1})+NH_{3}. Laser‐induced fluorescence and action spectroscopy have been combined with isotopic studies to map out the characteristics of these states. Both excited states are found to be bound by more than 5000 cm^{−} ^{1}, over 20 times greater than the ground state binding energy. Extensive vibrational structure is found and interpreted in terms of a stretching progression of the Hg–NH_{3}bond and bending of the NH_{3} moiety with respect to the mercury atom. The two states show striking differences in their behavior with respect to predissociation to Hg(6 ^{3} P _{0}). The B̃ state is not observed in fluorescence, but predissociates efficiently to Hg(^{3} P _{0})+NH_{3}, while the Ã state shows predominant fluorescence with only a minor amount of Hg(6 ^{3} P _{0}) formation. Rotational band contour analysis has been used to assign the B̃ state as the ^{3} E and the Ã state as the ^{3} A _{1} state. Both states are characterized by a shortening in the Hg–N bond distance from 3.35 Å in the ground state to about 2.2 Å in either excited state. The rotational contour assignments show that the electronic angular momentum of the excited mercury atom is preserved in the complex despite the complex’s polyatomic nature. This allows an interpretation of the electronic relaxation in a quasidiatomic fashion. All our results are consistent with a C _{3v } geometry for the Hg–NH_{3} complex in both the ground and excited states. The characteristics of the Ã and B̃ states and their couplings to the ã state correlated to ^{3} P _{0} enable a comparison with the full‐collision studies and has led us to postulate the Ã and B̃ states as the source of the luminescence observed in those studies.

Dependence of the N_{2} vibrational potential on density
View Description Hide DescriptionRecently, part of the vibrational spectrum of the ground electronic (X ^{1}∑^{+} _{ g }) state of condensed phase N_{2}, shocked to high density, has been measured by Schmidt, Moore, and Shaw. Densities (ρ) of nearly three times the ambient liquid value were obtained in the shock wave experiments. Due to the high temperatures achieved behind the shock waves, up to six vibrational levels of N_{2} were observed. These vibrational spectra show clear frequency shifts from their ambient condition values. Here, these experimental spectra are used to infer changes in the vibrational potential of N_{2} due to the high density environment. The vibrational portion of the Rydberg–Klein–Rees (RKR) method is used to do this. We find that, in the range of densities studied, the energies [E(n;ρ)] of the first five vibrational transitions of N_{2} can be accurately represented by E(n;ρ)=ω_{ e }(ρ)(n+ 1/2 )−ω_{ e } x _{ e }(n+ 1/2 )^{2}, where ω_{ e }(ρ)=ω^{0} _{ e }+A[1−(ρ_{0}/ρ)^{1} ^{/} ^{3}] and ω_{ e } x _{ e } is a constant which is independent of the thermodynamic state of the environment; here, ω^{0} _{ e } and A are fitting constants and ρ_{0} and ρ are the ambient (liquid) and shocked densities of N_{2}. Given E(n;ρ), one can obtain the classical turning point difference, r _{1} _{2}, of the N_{2} potential as a function of n and ρ by using the RKR procedure. It is found that at the highest shock density observed (2.13 g/cm^{3}), the relative change in r _{1} _{2} from the ambient condition values is about −1% for the first six vibrational levels.

Orientation‐selective ^{1} ^{4}N electron spin echo envelope modulation (ESEEM): The determination of ^{1} ^{4}N quadrupole coupling tensor principal axis orientations in orientationally disordered solids
View Description Hide DescriptionWe present an orientation‐selective ESEEM (electron spin echo envelope modulation) technique. From a measurement of the variation of the amplitudes of modulation components at ^{1} ^{4}N pure quadrupole frequencies as a function of irradiation position within an EPR(electron paramagnetic resonance)powder pattern, we determine the orientation of the principal axes of the quadrupole coupling tensor relative to the principal axes of the tensor which governs the dispersion of the EPR spectrum in an orientationally disordered sample. The pure quadrupole frequencies appear when the EPR frequency is selected such that the nuclear Zeeman and the hyperfine interaction are approximately equal in magnitude. We have applied the method to a mercaptoethanol complex of myoglobin in which the pure quadrupole frequencies originate from ^{1} ^{4}N in the proximal imidazole ring. Our results enable us to validate the assignment of the quadrupole modulations to the metal‐coordinated nitrogen of the imidazole ring, to correlate the principal axes with the principal values of the quadrupole coupling tensor, and to determine the orientation of the imidazole ring relative to the principal axes of the (low‐spin) Fe(III) g matrix. These findings are discussed and compared with results of previous studies.

Ring torsional dynamics and spectroscopy of benzophenone: A new twist
View Description Hide DescriptionThe low energy portion of the high resolution S _{1}←S _{0}fluorescence excitation spectrum of benzophenone recently reported by Holtzclaw and Pratt [J. Chem. Phys. 8 4, 4713 (1986)] is modeled here using a simple two‐degree‐of‐freedom vibrational Hamiltonian. The Hamiltonian features a 1:1 nonlinear resonance between the two low frequency ring torsional modes of the molecule in its S _{1} state. Line positions and intensities of the two major spectral progressions are well reproduced using parameters similar to those derived from earlier matrix diagonalizations. The comparison of the theory and experiment results in a determination of the displacement of the S _{1}surface relative to the ground electronic state along the symmetric torsional coordinate and permits a calculation of the excitation spectra of various isotopically substituted molecules not yet measured in the laboratory. A clear picture of the relationship between the dynamics on the S _{1}surface and the spectroscopy of benzophenone is revealed by comparing a time domain analysis of the experimental data with wave packet dynamics on the model S _{1}surface. This comparison provides new insight into energy flow in the isolated molecule and permits a qualitative simulation of the effects of collisional quenching on the fluorescencespectrum. We also discuss, using a classical trajectory analysis, the resonance dynamics of the torsional modes and note the existence of heretofore undetected local modes in the high resolution spectrum.

Structure and vibrational dynamics of the CO_{2} dimer from the sub‐Doppler infrared spectrum of the 2.7 μm Fermi diad
View Description Hide DescriptionSub‐Doppler infrared spectra of two Fermi resonance coupled bands of carbon dioxide dimer have been obtained at 3611.5 and 3713.9 cm^{−} ^{1} using an optothermal molecular beam color‐center laser spectrometer. The band origins for the complexes are red shifted by approximately 1 cm^{−} ^{1} from the corresponding ν_{1}+ν_{3}/2ν^{0} _{2}+ν_{3} CO_{2} bands. The higher frequency band is perturbed while the lower frequency band appears free of extraneous perturbations as determined from a precision fit to a Watson asymmetric rotor Hamiltonian. This fit and the observed nuclear spin statistical weights reveal that the complex is planar with C _{2h } symmetry. The C‐‐C separation and C‐‐C–O angle are determined to be 3.599(7) Å and 58.2(8)°, respectively. The nearest neighbor O‐‐C distance is 3.14 Å which is the same as that found in the crystal. From the centrifugal distortion analysis the weak bond stretching and symmetric bending frequencies are estimated to be 32(2) and 90(1) cm^{−} ^{1}. No interconversion tunneling is observed.

Near infrared spectroscopic observation of the linear and cyclic isomers of the hydrogen cyanide trimer
View Description Hide DescriptionSub‐Doppler resolution infrared spectra have been obtained for both the linear and cyclic conformers of the hydrogen cyanide trimer. In the case of the linear trimer, all three vibrational bands correlating with the C–H stretching fundamental of the hydrogen cyanide monomer (ν_{1}) have been observed. The vibrational predissociation lifetime of the complex is found to be strongly mode specific. For the cyclic trimer, which has only one (doubly degenerate) infrared allowed band associated with the C–H stretch, the rotational structure is characteristic of an oblate planar symmetric top. Molecular constants are reported for both conformers. In addition, several other bands are observed in the spectrum which, although not rotationally resolved, are tentatively assigned to the tetramer.

Semiclassical phase space evolution of Fermi resonance spectra
View Description Hide DescriptionThe evolution of the semiclassical phase space of a Fermi resonance spectrum is investigated as the strength of the resonance coupling is varied between zero and the strong coupling limit. The phase space evolution gives information beyond that contained in the phase space profile of the experimental spectrum alone. The zero‐order phase space is found to be different in important respects from that of the pendulum model of a nonlinear resonance. In the weak coupling regime, the phase space evolution is essentially like that of a dynamical barrier picture. In the strong coupling regime of ‘‘intrinsic resonance,’’ the phase space structure is much different. Topology change appears to take place in a more discontinuous manner than in the weak coupling regime. The phase space evolution shows that some levels are problematic for an adiabatic switching treatment. The origin of some anomalous levels seen both in phase space profiles of experimental spectra and in semiclassical quantization studies is clarified.

Coupling interaction between the sulfate ion ν_{2} modes and the lithium modes in LiKSO_{4}
View Description Hide DescriptionResonant couplings between the sulfate ion ν_{2} mode and the lithium ion translatory mode in E _{2} and E _{1} symmetries have been studied in single crystal^{7} LiKSO_{4} and ^{6} LiKSO_{4} from polarized Raman spectra between 298 and 519 K and from infrared reflectivityspectra at room temperature. Coupling effects are negligible in ^{7} LiKSO_{4} whereas in ^{6} LiKSO_{4} the lithiumisotope effect shifts the lithium translatory mode into resonant coupling with the sulfate ion ν_{2} mode. The resonant coupling mechanism between the two fundamental E _{2} modes is elucidated by a comparative analysis of the temperature dependence of the frequency shifts and the relative intensities. The coupling interaction in E _{1} symmetry can be only qualitatively described due to poor resolution of the Raman bands. It is possible to distinguish anharmonic processes which couple two modes from anharmonic processes which perturb each state individually. It was found that coupling anharmonicity decreased while perturbative anharmonicity increased with increasing temperature. Detailed consideration of various intensity ratios in E _{2} symmetry allows the separation of the isotope effect from the coupling contribution to the polarizability derivative tensor. In E _{1} symmetry TO–TO and LO–LO coupling interactions between the ν_{2} mode of the sulfate ion and the lithium translatory mode were deduced from the frequency and intensity data.

ESR investigation of the cation radicals ^{1} ^{4}N_{2} ^{1} ^{3}CO^{+}, ^{1} ^{5}N_{2} ^{1} ^{2}CO^{+}, and ^{1} ^{5}N_{2} ^{1} ^{3}CO^{+}: The trapping of ion–neutral reaction products in neon matrices at 4 K
View Description Hide DescriptionESR results are reported for the cation radicals ^{15} N_{2} ^{12} CO^{+} , ^{15} N_{2} ^{13} CO^{+} and ^{14} N_{2} ^{13} CO^{+} trapped as isolated ions in neon matrices at 4 K. The N_{2}CO^{+} radical was generated by codepositing N_{2} and CO into a neon matrix under ionizing conditions (both photoionization at 16.8 eV, and 50 eV electron bombardment). A complete resolution of the ^{14} N, ^{15} N, and ^{13} C Atensors reveal that the radical is planar and nonlinear (NNC O). Electronic structure changes that occur as N^{+} _{2} and CO (or CO^{+} with N_{2}) combine to form N_{2}CO^{+} are analyzed by comparing the nuclear hfs of the diatomic reactants with that of the product radical. The ^{13}C hfs is extremely large with A _{ x } =1376(1); A _{ y } =1407(1), and A _{ z } =1403(1) MHz. The Atensor for the inner ^{14} N atom is: A _{ x,y } =200.2(6) and A _{ z } =226.6(3) MHz. The outer ^{14} N has ‖A _{ x,y }‖ =4(1) and A _{ z } =9.4(2) MHz. The nuclear gtensor appears axially symmetric with g _{ x,y }=2.0007(3) and g _{ z } =2.0002(3). SCF calculations also show N_{2} CO^{+} to be nonlinear and yield A values in reasonably good agreement with experiment. These ESR results for N_{2}CO^{+} are compared with similar measurements for the isoelectronic ions C_{2}O^{+} _{2}, N^{+} _{4}, and C_{2}N^{−} _{2}.

Direct excitation studies of the diffuse bands of alkali metal dimers
View Description Hide DescriptionDirect dye laser excitations of the K_{2} yellow, Rb_{2} orange, and Cs_{2} near‐infrared diffuse bands have been investigated. Experimental results are shown to be consistent with the assumed bound–free 2 ^{3}Π_{ g }–1 ^{3}∑^{+} _{ u } excitations. It is found that for Rb_{2} and Cs_{2}, spin–orbit interactions become so significant that the 2 ^{3}Π_{ g } state is strongly split into three quite independent component states.

Optical spectra and energy level analysis of Dy^{3} ^{+}:LaCl_{3}
View Description Hide DescriptionThe absorptionspectrum of Dy^{3} ^{+}:LaCl_{3} at 4 K has been photographed and measured from 20 000 to 38 000 cm^{−} ^{1}. Based on this and previous data, an empirical energy level scheme consisting of 151 observed crystal levels from 0 to 34 130 cm^{−} ^{1} has been determined for the 4f ^{9} ground configuration of trivalent dysprosium in LaCl_{3} crystals. An extended Hamiltonian with 20 adjustable parameters is used to fit by least squares the observed levels with a mean error of 6.9 cm^{−} ^{1}.

High‐resolution photoionization spectrum of water molecules in a supersonic beam
View Description Hide DescriptionWe have obtained high‐resolution (∼1.5 cm^{−} ^{1}) photoionizationspectra of supersonically cooled (T _{rot}∼50 K) H_{2}O and D_{2}O in the 1000–900 Å range. The light source, which used the technique of frequency tripling in a pulsed free jet of gas, is described briefly. Spectra are rotationally resolved. Vibrationally excited autoionizing Rydberg series converging to the ground electronic [X̃; (1b _{1})^{−} ^{1}] state of the molecular ion are detected. This may well be the first example of a highly resolved Rydbergspectrum of a stable polyatomic molecule. From the convergence limit, the ionization potential H_{2}O is determined to be 101 777±7 cm^{−} ^{1}. Intensities of the Rydberg stateautoionization signals are smaller than predicted with known Franck–Condon factors, indicating that predissociation is a competitive decay channel. Rydberg state lifetimes are ∼1 ps, deduced from homogeneous linewidths. Autoionizing features from Rydberg states associated with the ion’s quasilinear Ã (3a _{1})^{−} ^{1} state are observed with linewidths above 10 cm^{−} ^{1}, indicating that their lifetimes are less than ∼0.5 ps. Rotational assignments of some of the bands in this linear←bent transition show that the Rydberg and ionic state geometries are nearly identical. A consistent assignment of the controversial bending (v _{2}) quantum number and Rydberg series quantum defect δ=−0.037 have been provided.