As the first attempt to determine the gas-phase structure of molecules forming liquid crystals, the molecular structure of p-azoxyanisole (PAA, CH 3 O-C 6 H 4 -NOdN-C 6 H 4 -OCH 3 ), a mesogen, has been studied by gas electron diffraction. A high-temperature nozzle was used to vaporize the sample. The temperature of the nozzle was about 170°C. Structural constraints were taken from HF/4-21G(*) ab initio molecular orbital calculations on PAA. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by normal coordinate analysis. The structural model assuming four conformers well reproduced the experimental data. Five bond distances, six bond angles, and two dihedral angles were determined. Mean amplitudes were adjusted in five groups. The dihedral angles between the phenylene rings and the azoxy plane have been determined to be 11(26)°and 11(11)°, and these values are in agreement with those in the solid phase determined by X-ray diffraction within experimental errors. The conformation of the core of this mesogen is mainly ascribed to the interaction between the π-electrons of the azoxy group and the aromatic rings.

Using the Green Bank 100 m telescope and the Nobeyama 45 m telescope, we have observed the rotational emission lines of the three 13 C isotopic species of HC3N in the 3 and 7 mm bands toward the low-mass star-forming region L1527 in order to explore their anomalous 12 C/ 13 C ratios.The column densities of the 13 C isotopic species are derived from the intensities of the J = 5-4 lines observed at high signal-to-noise ratios. The abundance ratios are determined to be 1.00:1.01 ± 0.02:1.35 ± 0.03:86.4 ± 1.6 for [H 13 CCCN]:[HC 13 CCN]:[HCC 13 CN]: [HCCCN], where the errors represent one standard deviation. The ratios are very similar to those reported for the starless cloud, Taurus Molecular Cloud-1 Cyanopolyyne Peak (TMC-1 CP). These ratios cannot be explained by thermal equilibrium, but likely reflect the production pathways of this molecule.We have shown the equality of the abundances of H 13 CCCN and HC 13 CCN at a high-confidence level, which supports the production pathways of HC3N via C2H2 and C2H2 + . The average 12 C/ 13 C ratio for HC3N is 77 ± 4, which may be only slightly higher than the elemental 12 C/ 13 C ratio.Dilution of the 13 C isotope in HC3N is not as significant as that in CCH or c-C3H2. We have also simultaneously observed the DCCCN and HCCC 15 N lines and derived the isotope ratios:[DCCCN]/[HCCCN] = 0.0370 ± 0.0007 and [HCCCN]/[HCCC 15 N] = 338 ± 12.

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The molecular structure of a typical mesogen, 4-methoxybenzylidene-4′-n-butylaniline (MBBA, CH 3 O-C 6 H 4 -CHdN-C 6 H 4 -(CH 2 ) 3 -CH 3 ), has been studied by gas-phase electron diffraction (GED). The nozzle temperature was about 150°C. Structural constraints in the GED data analysis were obtained by the ab initio MO calculation at the HF/4-21G(*) level of theory. Vibrational amplitudes and shrinkage corrections were calculated from the harmonic force constants given by a normal coordinate analysis. The phenylene ring attached to the C(dN) atom and the azomethine group (-CHdN-) are essentially on the same plane, i.e., the dihedral angle is 0(12)°. The phenylene ring bonded to the nitrogen atom is found to be out of the plane of the azomethine group and the determined value of the dihedral angle, 48(9)°, in the gas phase is larger than that in the crystalline state. This is mainly due to the steric interaction between the hydrogen atoms of the azomethine group and the phenylene ring. In the gas phase the four rotational conformers with respect to the configurations of the n-butyl group were assumed to exist. Their conformational abundance was fixed, as calculated from the ab initio relative energies. The principal bond distances and angles (r g /Å and ∠ R /deg) determined by GED are r(NdC) ) 1.290 (12), r(C-N) ) 1.413 (12), r(C az -C ring ) ) 1.467 (3), 〈r(C ring -C ring )〉 ) 1.400 (6), ∠C-NdC ) 119.0(18), ∠NdC-C ) 121.6(13), ∠NC ring C 14 ) 128.5(25), ∠C az C ring C 4 ) 121.2 (dependent), 〈CCC ring 〉 ) 120.0(3), 〈∠CCC butyl 〉 ) 116.2(11), ∠C 5 C ring O )129.3 (16), where C az , C ring , and C butyl denote the carbon atoms of the azomethine, phenylene and butyl groups, respectively. C 14 and C 4 are the C atoms of the rings synclinal to the C(dN) atom and cis to the H az atom, and C 5 is the C atom of the ring cis to the C atom of the methoxy group. The values in parentheses are three times the standard deviations. The notation 〈 〉 represents the average value. The transition temperature from the nematic to liquid phases was discussed on the basis of the determined molecular structure.

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Recoil-induced rotational excitation accompanying photoionization has been measured for the X, A, and B states of N(2)(+) and CO(+) over a range of photon energies from 60 to 900 eV. The mean recoil excitation increases linearly with the kinetic energy of the photoelectron, with slopes ranging from 0.73×10(-5) to 1.40×10(-5). These slopes are generally (but not completely) in accord with a simple model that treats the electrons as if they were emitted from isolated atoms. This treatment takes into account the atom from which the electron is emitted, the molecular-frame angular distribution of the electron, and the dependence of the photoelectron cross section on photon energy, on atomic identity, and on the type of atomic orbital from which the electron is ejected. These measurements thus provide a tool for investigating the atomic orbital composition of the molecular orbitals. Additional insight into this composition is obtained from the relative intensities of the various photolines in the spectrum and their variation with photon energy. Although there are some discrepancies between the predictions of the model and the observations, many of these can be understood qualitatively from a comparison of atomic and molecular wavefunctions. A quantum-mechanical treatment of recoil-induced excitation predicts an oscillatory variation with photon energy of the excitation. However, the predicted oscillations are small compared with the uncertainties in the data, and, as a result, the currently available results cannot provide confirmation of the quantum-mechanical theory.

In the photoelectron spectrum of N 2 the apparent ionization energy to form the B 2 ⌺ u + state increases linearly with the photon energy. Rotationally resolved measurements of the fluorescent decay of this state show a linear increase of rotational heating with increasing photon energy. These results are in quantitative agreement with the prediction of the theory of recoil-induced rotational excitation, indicating that the rotational heating that has been observed previously arises primarily from such recoil-induced excitation. Together with other results that have been reported they show that recoil-induced internal excitation is significant in many situations, including near threshold.When a photoelectron is ejected from an atom, molecule, or solid the remaining ion has a recoil momentum that is equal and opposite to that of the electron. Although the first discussions of this effect ͓1͔ were limited to translational recoil, it was recognized by Domcke and Cederbaum ͓2͔ that the recoil effect could lead to internal excitation of the ion. However, this prediction remained unverified until recent observations in core-electron photoelectron spectra of recoil excitation of vibrations in molecules ͓3,4͔ and phonons in solids ͓5͔. These experiments show that the recoil-induced internal excitation is quantitatively in accord with a model based on emission of the electron from a localized atom.Although a model based on emission from a localized atom may be appropriate for core ionization, it is not apparent that such a model is appropriate for valence ionization, where the electrons are delocalized. This question has been recently addressed by Takata et al. ͓6͔ who showed that at a photon energy of 8 keV there is a shift in the apparent position of the Fermi edge of aluminum that is consistent with the recoil being taken up by a single atom.The investigations mentioned above have been concerned with vibrational excitation. Here we consider recoil-induced rotational excitation during valence photoionization of N 2 . Thus we extend the previous investigations by considering a different type of internal excitation and by considering valence excitation in a distinctly different system ͑a small molecule rather than a solid͒. Specifically we investigate rotational excitation during photoionization to produce the B 2 ⌺ u + state of N 2 + . Using both photoelectron and fluorescence spectroscopy we show that there is recoil-induced rotational excitation in quantitative accord with a model based on emis-sion of the electron from a localized atom. Moreover, we note that this effect is observable even within 100 eV of threshold. Thus it becomes apparent that significant recoilinduced internal excitation is widespread in terms of both the physical system ͑molecule or solid͒ and the energy range.It has been previously noted that the distribution of rotational states produced during photoionization to form the B 2 ⌺ u + state of N 2 + depends on the photon energy ͓7,8͔. The distribution shifts to higher values of the rotational quan...

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