K.K. Innes

Reassignment of the “Forbidden” character in the Ã1B3u-X̃1Ag transitions of pyrazine-d0, 15N and d4

Abstract Contours of bands polarized in the plane of the pyrazine (1,4-diazine) molecule are analyzed in greater detail than before (J. Mol. Spectrosc. 11, 257 (1963); 21, 66 (1966)). Largely on the basis of simulations of the pyrazine-15N and d4 bands, it is concluded that the vibronic transition moment lies not along the line of the nitrogen atoms but perpendicular to that line. The reassignment requires the vibration responsible for the bands to be ν10a and the electronic transition(s) which loans intensity to be 1B2u. It is confirmed from the derived changes of inertial constants that the two nitrogen atoms lie closer together in the ecited state than in the ground state.

A low-lying excited electronic state of the AlO molecule and the ground-state dissociation energy

Abstract Emission bands near 2500 and 2800 A, from a microwave discharge through gaseous AlCl3 and O2, have been found to have a common lower 2Πi state. Rotational analysis of the 0-0 band of the 2800 A system, and also the vibrational analysis, show that the system originates from the 2Σ+ state at 40 300 cm−1 and terminates in the previously unrecognized 2Πi state at 5300 cm−1. In the 2500 A system the upper state is found to be a 2Δi state from the rotational analysis. The constants obtained for the new states are (cm−1): T 00 B 0 ω e ω e χ e 2 Δ i 45 260 0.4935 A 2 Π i 5282 0.5353 728.5 4.15 Establishment of the low-lying Π-state allows a reinterpretation of a continuum observed by Tyte, which gives an upper limit to the ground-state dissociation energy of 5.2 eV. This, in turn, suggests that the B2Σ+ state (at 2.5 eV) dissociates into ions (Al+ + 0−) 4.5 eV above the lowest free-atom energy.

The geometric structure of s-tetrazine and its change on electronic excitation

Abstract We report improved rotational constants that are consistent with recent low temperature, high resolution spectra of gaseous tetrazine as well as with older room temperature spectra. The constants are sufficient to overdetermine the geometries of the molecule in the ground state and in the A 1 B 3u excited state. The ground state results agree with those of X-ray diffraction. Moreover, earlier disagreements with results of Franck-Condon treatments about the effects of electronic excitation on the tetrazine geometry are resolved. We append an easy and general procedure for calculation of nuclear spin statistical weight factors.

Molecular electronic spectroscopy by Fabry-Perot interferometry. Effect of nuclear quadrupole interactions on the line widths of the B3Πo+-X1Σg+ transition of the I2 molecule

Abstract A pressure-scanning Fabry-Perot interferometer spectrometer was tested by recording the 5461 A atomic lines of mercury in absorption in a radio-frequency discharge. The smallest half-width (full width at half height) measured was 0.024 cm−1. Assuming the indicated resolving power of 750 000, we studied the 18-0 and 20-1 vibrational bands near 5650 A and the 30-0 band near 5350 A in the B 3 Π 0 + -X 1 Σ g + transition of iodine vapor at a pressure of about 10 μ. Instead of lines sharper than those of mercury, lines at least 25% broader were found. Moreover, the half-width alternates with the rotational quantum number J, being about 0.030 cm−1 for even J″ and 0.035 cm−1 for odd J″. These widths are explained in detail by a nuclear electric quadrupole interaction in both states, with a difference in coupling constants of |eQq′ − eQq″| = 0.070 cm−1. The result is confirmed by our studies of 129I2 line widths and agrees well with a recent interpretation of saturated absorption by one rotational line in the 11-5 band of the same transition.

Assignments of visible absorption of s-tetrazine vapor

Abstract A detailed vibrational analysis of the 5515 A absorption of s -tetrazine (tetra-azabenzene) vapor is attempted. More than 80% of the total intensity of the system can be accounted for by band assignments to a single allowed electronic transition, 1 B 3 u - 1 A g . The only progression-forming vibration in absorption in ν 6 a . Three progressions in ν ′ 6 a account for all intense cold bands. Isotopic shifts of bands lead to convincing assignments of the two excited-state vibrations upon which ν 6 a progressions are based, namely ν 6 b and ν 8 a . It is shown that in all probability the antisymmetric mode ν 6 b is prominent because of vibrational (Fermi) resonance rather than because of vibronic interactions.

A triplet-singlet transition of s-tetrazine

Abstract A very weak but sharp absorption system of s -tetrazine vapor has been found to extend from 8000 to 6000 A. It is characterized by a single progression of bands whose relative intensities require an appreciable change of geometric structure, confined mainly to the NN bonds and NCN angles. A 134-m absorbing path reveals rotational structure in the OO band at 13 608 cm −1 . Analysis of the structure is consistent with one of the two geometry changes allowed by the vibrational analysis, namely, Δ r( NN ) = −0.06 A and Δ NCN = +3.4° . Intensity alternation in the rotational structure shows that the excited state is triplet. Since the five observed vibrational frequencies of the new state have nearly the same respective values as those of the 1 B 3 u state at 18 128 cm −1 , it is assumed that the orbital configuration of the excited state also is the same, that is, the excited state is 3 B 3 u .

The ã←X˜1A1 and A˜1B1 ←X˜1A1 electronic transitions of pyridazine-d0, -d2, and -d4 vapors

Abstract High resolution absorption spectra of 1,2-diazine and two deuterated modifications have been measured between 3000Aand 5000A, and analyzed. Two excited electronic states, a singlet and a triplet, have been identified. Rotational analyses of the 0-0 bands of the two systems have shown the systems to be 1 B 1 - 1 A 1 (3700A) and 3 A 2 - 1 A 1 or 3 B 1 - 1 A 1 (4450A). It has been found that the sharp rotational structure of the 1 B 1 - 1 A 1 0-0 band of pyridazine- d 0 indicates a condition on the change of inertial constants not identified before for planar asymmetric rotors, namely, C′ - C″ =B¯′ -B¯″ (where B¯=( A + B )2). Changes in A , B , and C found by contour analysis prove that the molecule is elongated by an (atomic) average of about 10% along a direction perpendicular to the symmetry axis. This considerable change of geometry is reflected in the Franck-Condon contour of vibrational structure of the singlet system. Isotope effects are used to demonstrate that the extreme complexity of vibrational structure on the high-frequency side of the origin arises from large and extensive Fermi (anharmonic) resonances, linking ν′ 6a and the overtone of an antisymmetric mode thought to be ν 16b . It has followed that identification of the active vibrations has had to be based mainly on observations of their less perturbed lower-state counterparts at frequencies as far as 2400 cm −1 below that of the origin. A unique feature is the large reduction of ν 6a from 636 to 374 cm −1 (pyridazine- d 4 ) on electronic excitation. That reduction, in addition to the large anharmonic term in the potential energy of the 1 B 1 state and intensity stealing by the totally-symmetric mode, ν 1 , implies the presence of a second 1 B 1 state, nearby, but not recognized directly.

Molecular electronic spectroscopy by Fabry-Perot interferometry: Contour analysis of a band of the 5500 Å absorption system of s-tetrazine vapor

Abstract The very sharp, rotational fine structure of the s-tetrazine-d0 type-C absorption band at 18913 cm−1 has been recorded with a Fabry-Perot interferometer spectrometer for which the instrumental resolving power is 1 300 000. Because of the absence of spectrometer slit distortion, it has been possible to interpret in detail not only peak positions but also their shapes half-widths and relative heights. Molecular parameters that have been well determined by this asymmetric rotor band contour analysis are the band center and five moments of inertia.

DIMETHYL-S-TETRAZINE VAPOR: VISIBLE ABSORPTION, LASER-EXCITED FLUORESCENCE, AND PHOTOPHYSICS

Abstract Except for broader bands, hot bands, and a 500-cm−1 shift of the 0-0 band to higher frequencies, dimethyl-s-tetrazine as a vapor is found to exhibit the same visible absorption and excited-state zero-level emission spectra that it does when dissolved in p-xylene at 4.2 K and below. Vibrational frequencies in both A 1 B 3u and X 1 A g states show almost no solvent effects. Even an unusual “negative” anharmonicity of the principal progression-forming mode ν6a in the ground state is the same in both phases. Franck-Condon activity also is independent of phase; in fact, for ν6a it is the same as for s-tetrazine in the vapor phase. Single vibronic level excitations into the 0-0, 6a01, 101, and 6a02 bands show that vibrational relaxation is appreciable for pressures of a Torr and greater. Mode-to-mode flow does not follow the propensity rules exhibited by the S1 state of benzene. Rotational relaxation and competition of collisional relaxations with predissociation are revealed when foreign gas is added.

Two 0-0 bands in the 3700-A (π∗ ← n) absorption of pyridazine?

Abstract Four type-C bands of the 3700-A, A 1 B 1 - X 1 A 1 transition of pyridazine (1,2-diazine) have been subjected to rotational contour analysis. Two equally good computer simulations of the 0-0 band (26 648 cm−1) differ only by an interchange of direction of the axes of largest and intermediate moments of inertia of the excited state. Either solution implies that the effect of electronic excitation is to elongate the molecule along the direction perpendicular to the symmetry axis. A similar effect of excitation of one quantum of ν″6a is found. Contour analysis of a band at 27 021 cm−1 reveals a large, negative inertial defect for the upper level, which is confirmed by analysis of a hot band connecting with the same level. The defect is discussed as a clue to the long-standing problem of vibrational complexity in the 3700-A absorption. Of the three interpretations discussed, the one most consistent with all published experimental data is that the 27 021-cm−1 band is the 0-0 band of the B 1 B 1 - X 1 A 1 transition.