Thioformaldehyde: Vibrational analysis of the Ã1A2-X̃1A1 visible absorption system

The electronic absorption spectra of thioformaldehyde and thioformaldehyde-d₂ have been obtained. A vibrational analysis of the discrete band system in the 6100-4400-Å region is reported. The type A origin bands are at $\text{16 394}\text{16 484}\text{cm}{}^{\mathrm{?1}}$ for $\text{CH}{}_2\text{S}\text{CD}{}_2\text{S}$, and are magnetic dipole allowed. The electronic transition is $\text{A}\text{?}{}^1\text{A}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ under the C_2v point group. Most of the intensity of the system is in type B bands, and is due to vibronic mixing with higher ¹B₂ states when the inversion mode ν₄ is excited. The molecule in the excited ¹A₂ state is “floppy-planar,” having a broad potential function with a barrier of the order of 20 cm^?1 to the inversion motion.     

Infrared diode laser spectroscopy of the NS radical

The v = 1 ← 0 vibration-rotation bands of the NS radical in the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electronic states were observed by using a tunable diode laser. From the least-squares analysis the band origins were determined to be 1204.2755(12) and 1204.0892(19) cm^?1, respectively, for $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. The rotational and centrifugal distortion constants and the internuclear distance in the X²Π electronic state were obtained as follows: B_e = 0.775549(10) cm^?1, D_e = 0.00000129(33) cm^?1, and $\text{r}{}_{\mathrm{e}}\text{= 1.49403(4)}\text{A}\text{?}$, with three standard deviations indicated in parentheses.     

Rotationally resolved infrared-ultraviolet double resonance spectroscopy in molecular D2CO and HDCO

Rotationally resolved infrared-ultraviolet double resonance (IRUVDR), consisting of sequentially excited rovibrational and rovibronic transitions sharing a common intermediate molecular level, is demonstrated. The technique employs pulsed CO₂ and tunable dye lasers and is applied to the molecules D₂CO and HDCO. For D₂CO, infrared pumping produces 100fold population enhancement in specific rotational sublevels of the v₄ = 1 level of the $\text{X}\text{?}{}^1\text{A}{}_1$ electronic ground state, followed by ultraviolet excitation in the 365-nm 4₁⁰ band of the $\text{A}\text{?}{}^1\text{A}{}_2\text{←}\text{X}\text{?}{}^1\text{A}{}_1$ electronic system. This ultraviolet excitation occurs at a specific set of dye laser frequencies, determined by the preceding rovibrational transition, and is detected by molecular fluorescence in the visible region. Similar effects observed in HDCO involve rovibrational pumping to either the v₆ = 1 or v₅ = 1 levels and give rise to enhanced rovibronic transitions in the 6₁⁰ and 5₁⁰ bands of the $\text{A}\text{?}\text{←}\text{X}\text{?}$ system, respectively. The resulting IRUVDR spectra enable detailed spectroscopic assignments to be made and are consistent with previous results from infrared and ultraviolet absorption, laser Stark, and infrared-radiofrequency double resonance spectroscopy. Collision-induced satellite structure, arising from rotational relaxation of the intermediate rovibrational level in the IRUVDR sequence, is also reported.     

The Ã1B2-X̃1A1 two-photon fluorescence excitation spectrum of 1,3-difluorobenzene

The $\text{A}\text{?}{}^1\text{B}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ system of 1,3-difluorobenzene has been observed using the technique of two-photon fluorescence excitation obtained with a pulsed dye laser. Calibration was achieved by a combination of the neon optogalvanic spectrum and etalon fringes. In circular, compared to linear, polarization the bands divide into two groups, those which are B₂-A₁ and which retain their intensity with circular polarization, and those which are A₁-A₁ and lose about 60% of their intensity under the same conditions. These two kinds of bands also show characteristic rotational contours. All of the A₁-A₁ bands whose assignments are established obtain their intensity through vibronic interaction in which the vibration ν′₂₅ (ν′₁₄ in the Wilson numbering) mixes the $\text{A}\text{?}$ with, presumably, the $\text{X}\text{?}$ state. There is an important Fermi resonance between the 9¹ and 10¹11¹ levels. Parts of the one-photon absorption spectrum have been photographed to identify sequences associated with the 0₀⁰ band for comparison with those observed in the two-photon spectrum, and to search for bands involving odd quanta of b₂ vibrations, including ν′₂₅ (ν′₁₄); none was found.     

The dipole moment of thioformaldehyde in its singlet and triplet π1 − n excited states

Laser-induced fluorescence excitation has been used to measure Stark splittings of selected lines in the $\text{A}\text{?}{}^1\text{A}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ and $\text{a}\text{?}{}^3\text{A}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_2$ band systems of H₂CS in electric fields up to 13 kV/cm. The derived excited state a-axis dipole moments are 0.820 ± 0.007 D for the 4¹ level of the ¹A₂ state; 0.838 ± 0.008 D for the zeroth vibrational level of ¹A₂; and 0.534 ± 0.015 D for the zeroth vibrational level of the ³A₂ state. These results are compared with the corresponding values of H₂CO, and interpreted in terms of the changing localization of the π and $\text{π}{}^1$ orbitals accompanying electronic excitation.     

Further studies of the electronic and vibrational spectra of ND2: Improved molecular parameters for the 000 and 010 levels of the ground state

Improved molecular parameters for the 000 and 010 levels of the $\text{X}\text{?}{}^2\text{B}{}_1$ ground state of ND₂ have been obtained by a weighted least-squares treatment using published microwave-optical double resonance frequencies, laser magnetic resonance data, infrared-optical double resonance frequencies, and extensive new optical data for the $\text{A}\text{?}{}^2\text{A}{}_1\text{-}\text{X}\text{?}{}^2\text{B}{}_1$ absorption spectrum. Revised assignments are given for 13 LMR transitions, including 4 “hot” band transitions.     

The 2780 Å absorption system of thiophosgene

The vapor phase absorption spectrum of thiophosgene (Cl₂CS) in the 2500–2900 Å region consists of a broad intense band ($\text{log}\text{?}{}_{\mathrm{<mtext>max</mtext>}}\text{= 3.5}\text{at}\text{2540}\text{A}\text{?}$. On the red side of this a vibrationally discrete structure is found which becomes increasingly diffuse and merges into the broad band as the wavelength is decreased. It is shown that this vibrational structure can be explained as due to a $\text{π → π}{}^1$, ${}^1\text{A}{}_1\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ electronic transition between a planar ground state and a pyramidal excited state of the molecule. In the latter state, the CS stretching mode ν₁′(a₁) = 681 cm^?1 and the CCl bending mode ν₃′(a₁) = 147 cm^?1. From the inversion doublet splitting of the out-of-plane mode ν₄′(b₁), the barrier to inversion is calculated to be ～126 cm^?1, with an equilibrium out-of-plane angle of ～20°.     

Thioformaldehyde: Rotational analyses of the Ã1A2-X̃1A1 visible absorption system

The $\text{A}\text{?}{}^1\text{A}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ electronic absorption spectra of CH₂S and CD₂S have been photographed under high resolution. Selected bands have been rotationally analyzed by least squares line fitting and by band contour methods. Improved rotational constants have been obtained for the ground states of CH₂S and CD₂S by use of combination differences. Bands of all three polarizations appear in the electronic spectrum. The type A origin band is magnetic dipole allowed, whereas the 4₀¹ band is type B. Perturbations are identified in the 0₀⁰ and 3₀¹4₀³ bands of CH₂S. The rotational constant A in the upper state decreases rapidly, in accordance with theoretical calculations, as successive quanta of the inversion mode ν′₄ are excited. The planar inertial defect has a small positive value in the zero level of the upper state although the molecule is slightly nonplanar; the r₈ geometry is $\text{r(}\text{CH}\text{) = 1.082}\text{A}\text{?}$, $\text{r(}\text{CS}\text{) = 1.701}\text{A}\text{?}$, angle HCH = 120°, and the out-of-plane angle is approximately 10°.     

The ã3A2 ← X̃1A1 absorption spectrum of thioformaldehyde: A vibrational and rotational analysis

The results of a vibrational and rotational analysis of the banded $\text{a}\text{?}{}^3\text{A}{}_2\text{←}\text{X}\text{?}{}^1\text{A}{}_1$ transition in $\text{CH}{}_2\text{S}\text{CD}{}_2\text{S}$ are presented. Only three of the six vibrational modes are active in the spectrum with $\text{ν′}{}_2\text{=}\text{1320}\text{1012}$, $\text{ν′}{}_3\text{=}\text{859}\text{798}$, and $\text{2ν′}{}_4\text{=}\text{711}\text{516}\text{cm}{}^{\mathrm{?1}}$. The spin forbidden transition gains intensity primarily by a mixing of the ${}^1\text{A}{}_1\text{(π}{}^1\text{,π)}$ and ${}^3\text{A}{}_2\text{(π}{}^1\text{,n)}$ states. This is confirmed by a rotational analysis of the 0₀⁰ band of both isotopes. The rotational analysis shows that the coupling in the $\text{a}\text{?}{}^3\text{A}{}_2$ state is near Hund's case b and that the spin constants are nearly 10 times greater than those observed for CH₂O. A $\text{CNDO}\text{2}$ calculation shows that this difference is due to the greater spin orbit coupling of S in CH₂S and to the smaller energy differences between the $\text{B}\text{?}{}^1\text{A}{}_1\text{(π}{}^1\text{,π)}$, $\text{b}\text{?}{}^3\text{A}{}_1\text{(π}{}^1\text{,π)}$, $\text{X}\text{?}{}^1\text{A}{}_1$, and the $\text{a}\text{?}{}^3\text{A}{}_2\text{(π}{}^1\text{,n)}$ states. The r₀ structure calculated from the rotational constants is $\text{r}{}_{\mathrm{<mtext>CS</mtext>}}\text{= 1.683}\text{A}\text{?}$, $\text{r}{}_{\mathrm{<mtext>CH</mtext>}}\text{= 1.082}\text{A}\text{?}$, β_HCH = 119.6°, and α (out of plane) = 16.0°. A simultaneous fit of the vibrational levels in ν′₄ of CH₂S and CD₂S to a double minimum potential function yielded a barrier to molecular inversion of 13 cm^?1 and an equilibrium out-of-plane angle of 15°.     

V.U.V. Excitation of benzene: C6H6 fluorescence and phosphorescence in rare gas and nitrogen matrices

Benzene isolated in rare gas or nitrogen matrix at about 5 K has been irradiated with photons of 4, 6, 8.4, and 10 eV energy. The phosphorescence $\text{a}\text{?}{}^3\text{B}{}_{\mathrm{1u}}\text{→}\text{X}\text{?}{}^1\text{A}{}_{\mathrm{1g}}$ and the fluorescence $\text{A}\text{?}{}^1\text{B}{}_{\mathrm{2u}}\text{→}\text{X}\text{?}{}^1\text{A}{}_{\mathrm{1g}}$ are observed amost in every case, from the vibrationally relaxed states. For both emissions, shifts are observed from one matrix to another. They depend upon the vibronic bands in the fluorescence case. Some features of spectra are interpreted as phonon-induced, specially the 0-0 bands electronically forbidden for both transitions. The phosphorescence versus fluorescence ratio increases when passing from a nitrogen matrix to a xenon matrix and as the photon energy increases. To account for these observations, we are led to believe that the $\text{a}\text{?}{}^3\text{B}{}_{\mathrm{1u}}$ state is not only populated from the $\text{A}\text{?}{}^1\text{B}{}_{\mathrm{2u}}$ state, but through upper singlet and (or) triplet states.     

Rotational analysis of the NO2 6125-Å region

In the NO₂ 6125-Å region 135 transitions belonging to four bands have been assigned using laser-induced fluorescence. On the whole, the rotational structure of the four vibronic bands is well characterized by near-prolate symmetric top relations. The 6125 and 6126-Å bands perturb each other, the former having predominantly the character of the $\text{A}\text{?}{}^2\text{B}{}_2$ state, the latter the $\text{X}\text{?}{}^2\text{A}{}_1$ state; this parent-daughter relationship is recognized to occur else-where in the NO₂ visible spectrum.     

New S1 vibronic level assignments for s-tetrazine vapor: Polarization and lifetime measurements

New fluorescence excitation and dispersed SVL fluorescence spectra of s-tetrazine vapor in supersonic expansions of helium and argon are reported. A forbidden in-plane-polarized component of the $\text{A}\text{?}{}^1\text{B}{}_{\mathrm{3u}}\text{-}\text{X}\text{?}{}^1\text{A}{}_{\mathrm{g}}$ transition is discovered at (0, 0) + 578 cm^?1 with a type-B band contour in rotationally resolved excitation spectra obtained with a single-frequency cw ring dye laser. Linewidth measurements of single rovibronic transitions provide data to calculate lifetimes of low-lying S₁ vibronic states of the isolated molecule. Depending on the vibrational mode involved, the lifetime varies from 0.05 to greater than 1 nsec. The number of cold-band assignments in the absorption spectrum of s-tetrazine vapor now confirmed by analysis of SVL fluorescence spectra increases from three to ten.     

The 370-nm electronic spectrum of tropolone: Evidence from single vibronic level fluorescence spectra regarding the assignment of some vibrational fundamentals in the X̃ and à states

Single vibronic level fluorescence (SVLF) spectra of tropolone from vibronic levels in the $\text{A}\text{?}{}^1\text{B}{}_2$ electronic state, in combination with recently reported supersonic jet spectra, have enabled the assigning of many absorption bands in the region of 0₀⁰ which had previously been impossible. Some of the complexity in these bands has been shown to be due to a large Duschinsky effect involving the two lowest b₁ vibrations, ν₂₅ and ν₂₆. It has been shown that these vibrations have wavenumbers of 176 and 110 cm^?1, respectively, in the $\text{X}\text{?}$ state, and 172 and 39 cm^?1 in the $\text{A}\text{?}$ state. This last value shows how unresistent the molecule is in the $\text{A}\text{?}$ state to out-of-plane bending in the region of the five-membered ring. Other aspects of the vibrational complexity are due to the effect of ν′₂₆ in increasing the barrier to tunnelling of the hydrogen-bonding proton in the $\text{A}\text{?}$ state contrasting with very little effect of ν″₂₆ in the $\text{X}\text{?}$ state.     

Infrared diode laser spectroscopy of the CF radical

The fundamental bands of the CF radical in the $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and $\text{X}{}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electronic states were observed by using an infrared tunable diode laser as a source. Zeeman modulation could be used in detecting lines not only in the ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ state, but also in ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, because the CF radical deviates considerably from Hund's case (a). From the least-squares analysis of the observed spectra, the following molecular constants were obtained: B_e = 1.416 704 (37) cm^?1, α_e = 0.018 419 (50) cm^?1, $\text{r}{}_{\mathrm{e}}\text{= 1.271 977 (17)}\text{A}\text{?}$, D_e = 6.68 (15) × 10^?6cm^?1, p₀ = 0.008 580 (21) cm^?1, p₁ = 0.008 52 (11) cm^?1, and $\text{ν}{}_0\text{= 1286.1281 (5)}\text{cm}{}^{\mathrm{?1}}$, with three standard errors in parentheses.     

Vibronic coupling as a major cause of the anharmonicity of antisymmetric stretching in the ground state of NO2

Approximate experimental and theoretical information about vibronic coupling of the $\text{X}\text{?}{}^2\text{A}{}_1$ (ground) and $\text{A}\text{?}{}^2\text{B}{}_2$ electronic states of NO₂—by its antisymmetric vibration ν₃(b₂)—is tested in model calculations of the accurately known ground-state levels ν₃ = 0, 1, 2, 3. The test is positive and it is estimated that 64% of the very large observed anharmonic constant χ₃₃ has its origin in vibronic coupling. In this model, ν₃ in the à state is predicted at about 1200 cm^?1.     

The electronic isotope shift in the A2Π-X2Σ+ bands of BeH,BeD and BeT

The 0-0, 1-1, 2-2, and 3-3 bands of the A²Π-X²Σ⁺ transition of the tritiated beryllium monohydride molecule have been observed at 5000 Å in emission using a beryllium hollow-cathode discharge in a He + T₂ mixture. The rotational analysis of these bands yields the following principal molecular constants.

$\text{A}{}^2\text{Π:}\text{B}{}_{\mathrm{e}}\text{= 4.192}\text{cm}{}^{\mathrm{?1}}\text{; r}{}_{\mathrm{e}}\text{= 1.333}\text{A}\text{?}$

$\text{X}{}^2\text{Σ:}\text{B}{}_{\mathrm{e}}\text{= 4.142}\text{cm}{}^{\mathrm{?1}}\text{; r}{}_{\mathrm{e}}\text{= 1.341}\text{A}\text{?}$

$\text{ω}{}_{\mathrm{e}}\text{′ ? ω}{}_{\mathrm{e}}\text{″ = 16.36}\text{cm}{}^{\mathrm{?1}}\text{; ω}{}_{\mathrm{e}}\text{′X}{}_{\mathrm{e}}\text{′ ? ω}{}_{\mathrm{e}}\text{″X}{}_{\mathrm{e}}\text{″ = 0.84}\text{cm}{}^{\mathrm{?1}}$

From the pure electronic energy difference (E_Π - E_Σ)_BeT = 20 037.91 ± 1.5 cm^?1 and the corresponding previously known values for BeH and BeD, the following electronic isotope shifts are derived

$\text{ΔE}{}_{\mathrm{e}}{}^{\mathrm{i}}\text{(}\text{BeH}\text{?}\text{BeT}\text{) = ?4.7 ≠ 1.5}\text{cm}{}^1\text{, ΔE}{}_{\mathrm{e}}{}^{\mathrm{i}}\text{(}\text{BeH}\text{?}\text{BeT}\text{) = ?1.8 ≠ 1.5}\text{cm}{}^1$

and related to the theoretical approach given by Bunker to the problem of the breakdown of the Born-Oppenheimer approximation.     

Matrix isolation spectrum of the 2491 Å system of NO2

The absorption spectrum of the 2491 Å NO₂ bands (${}^2\text{B}{}_2\text{←}\text{X}\text{?}{}^2\text{A}{}_1$) has been observed in neon and argon matrices at 6 K. Evidence for two distinct matrix sites is confirmed by the doubling of the electronic origin. The bands are shifted slightly to the blue (～ + 60 cm^?1) in neon and to the red (～ ?64 cm^?1) in argon. The excited state vibrational frequencies are reported.     

The two-photon excitation spectrum of fluorobenzene vapor

The two-photon excitation spectrum of fluorobenzene vapor has been recorded in the region of the $\text{A}\text{?}{}^1\text{B}{}_2\text{←}\text{X}\text{?}{}^1\text{A}{}_1$ transition. The spectrum shows considerable rovibronic structure with the bulk of the intensity lying in the subsystem induced by the ν₁₄(b₂) vibration. Two types of rovibronic contours, arising from ΔK_a = ±1 and from ΔK_a = 0, ±2 transitions, are identified. Major features in these contours are assigned by comparison with synthetic spectra, calculated using known upper and lower state rotational constants. The intensity distribution among the various bands in progressions of the totally symmetric vibrations is considerably different from that in the one-photon absorption spectrum, and the possible reasons for this are discussed.     

Why are formaldehyde and (possibly) propynal nonplanar in their S1(π1, n) electronic states?

Abnormally low frequencies observed for the out-of-plane vibration (b₁) of the $\text{A}\text{?}{}^1\text{A}{}_2$ electronic state of formaldehyde (H₂CO) and for the analogous carbonyl hydrogen vibration (a″) of $\text{A}\text{?}{}^1\text{A″}$ propynal (HCCCHO) are modeled by means of two-state calculations of vibronic coupling with higher singlet states, ¹B₂ and ¹A′, respectively. In each case, the active vibration is the out-of-plane hydrogen motion. The same vibronic calculations reproduce also the large positive anharmonicities of the active vibrations in the $\text{A}\text{?}\text{(π}{}^1\text{, n)}$ states; for H₂CO the calculated vibrational spacing alternates as observed, consistent with the known nonplanar structure, while in propynal the calculated spacing increases regularly, thus predicting an effectively planar structure. The nonplanarity of H₂CO is caused mainly by a vibronic coupling constant nearly twice that of propynal. The H₂CO coupling constant is near the value estimated independently by means of the intensity “borrowed” by the S₁-S₀ transition from the much stronger S₂-S₀ transition. Brief consideration is given to analogous vibrational levels of the ¹A₂ state of H₂CS and of the ³A₂ state of D₂CO in the vibronic context of this paper.     

Zeeman effects in the visible 2B2 ← 2A1 system of NO2

The Zeeman effect facilitates certain rotational assignments in bands of the ${}^2\text{B}{}_2\text{-}{}^2\text{A}{}_1$ electronic system of NO₂. A partial analysis of two bands of this system is reported.