1Ag Transition Research Articles

Measurement of static polarizabilities on C1H8 and C1D8

Laser Stark spectroscopy has been performed on C10H8 and C10D8 molecules in a collimated, seeded supersonic beam. The Stark splittings and shifts of many rotational lines in the 810 band of the A 1B3u←X 1Ag transition of naphthalene–H8 and –D8 could be measured in a static homogeneous electric field. The determined polarizability anisotropies of the A 1B3u state of C10H8 are P′aa−Pbb=9.9(2) Å3 and P′bb−Pcc=6.4(2) Å3, whereas the X 1Ag ground state results are P″aa−P″bb=6.2(2) Å3 and P″bb−P″cc=7.2(2) Å3. The change of the average polarizability between these states was determined to ΔP=P′−P″=1.6(1) Å3. The corresponding values of C10D8 were found to be P′aa−P′bb=10.1(2) Å3, P′bb−P′cc=6.2(2) Å3, P″aa−P″bb=6.1(2) Å3, P″bb−P″cc=7.3(2) Å3 and ΔP=P′−P″=1.6(1) Å3.

Measurement of static polarizabilities on s-tetrazine

Laser Stark spectroscopy has been performed on s-tetrazine molecules in a collimated, seeded supersonic beam. The Stark splittings and shifts of the rR(0)00 and the pP(1)10 lines in the vibrationless A 1B3u←X 1Ag transition could be measured in a static homogeneous electric field. The determined polarizability anisotropy of the A 1B3u state is 1/2(P′aa+Pbb)−P′cc =6.7(2) Å3, and of the X 1Ag ground state 1/2(P■aa+P″bb) −P■cc=0.8(2) Å3. The difference in the average polarizability P=1/3(Paa+Pbb+Pcc) between these two states was found to be ΔP=P′−P″=1.4(2) Å3.

Two-photon transitions of Ag−centers in KBr

Abstract Two-photon excitation spectra of Ag− centers in KBr have been measured. The main excitation peak appears only with linearly polarized excitation and has been assigned to the 1A1g → 1A1g transition. Weaker structures have a polarization dependence corresponding to 1A1g → 1E g and 1A1g → 1T2g transitions.

Two-photon spectroscopy of Rydberg states of jet-cooled C2H4 and C2D4

Spectroscopic studies of two-photon resonant vibronic bands of the (π,3s), (π,3p), and (π,nf ) (n=4–18) Rydberg states of jet-cooled C2H4 and C2D4 are reported. New vibronic bands are resolved by these low temperature (<10 K) observations and previous vibronic assignments of two-photon 3s and 3p transitions are confirmed. A new vibronic progression is observed in the 3s spectrum, tentatively assigned to two-photon allowed 1B2g←1Ag transitions, vibronically induced via excitation of the b1u ν6 CH2 antisymmetric scissors mode. Polarization ratio measurements and identification of the previously unreported 3pσ origin band show that the energies of the 3p levels are in good agreement with theoretical predictions. Higher-lying members of the s- and p-Rydberg series are not observed, suggestive of the onset of strong predissociation. These first observations of the two-photon allowed nf-Rydberg series yield ionization potential estimates of 84 799±5 cm−1 for C2H4 and 84 918±5 cm−1 for C2D4 that compare favorably with previous measurements.

Lowest energy excited singlet state of isolated c i s-hexatriene

In a previous letter [J. Chem. Phys. 92, 4622 (1990)] we reported the first observation of the 2 1Ag state of cis-hexatriene in a supersonic jet expansion by using resonance enhanced multiphoton ionization spectroscopy. Here, the vibrational analysis of the 1 1Ag→2 1Ag excitation spectrum of cis-hexatriene is presented. The excitation spectrum shows that cis-hexatriene in the 2 1Ag state deviates slightly from planarity; a conclusion which is corroborated by ab initio calculations indicating that the nonplanarity primarily involves the terminal hydrogen atoms. Except for observable intensity in the low frequency modes associated with the small out of plane distortion, the vibronic development of the 1 1Ag→2 1Ag transition in cis-hexatriene is similar to that observed for other polyenes: the 0–0 transition is the most intense feature and the next most intense band is the C■C stretching fundamental. Thus the general features of the electronic structure of the cis-hexatriene 2 1Ag state are analogous to those of other polyenes.

Splitting of the 2nd d–d Band of trans-[CoCl2N4]X-Type Complexes Observed in the Solid State at 113 K

Abstract The electronic spectra of trans-[CoCl2(NH3)4]Cl and its analogues measured in the solid state at 113 K show four ligand field bands assignable to the Ia(1Ega← 1A1g), Ib(1A2g← 1A1g), IIa(1B2g← 1A1g), and IIb(1Egb← 1A1g) transitions in D4h approximation. The ligand field parameters obtained from the observed data are; eσ(N)=7400–8000 cm−1, eσ(Cl)=4500–5400 cm−1, eπ(Cl)=300–830 cm−1, and the Racah’s parameter C estimated with the assumption of C=4B is ca. 2000 cm−1. A nearly linear relationship is obtained between the eσ(Cl) and eπ(Cl) values; the stronger the coordination of Cl− to Co(III), the stronger the π interaction of Cl− with Co(III).

Harmonic two-photon intensity sum rule for benzene b 2u modes

A harmonic two-photon cross section sum rule for benzene b 2u modes (ν 14 and ν 15) in the à 1B 2u←X̃ 1A 1g transition is derived. An approximate (but accurate) form of this rule based on skeletal mode displacements is used to obtain absorptivity values for C 6H 6 14 1 0 (6.3×10 −50 cm 4 s mol −1 photon −1) and 15 1 0 (1.6×10 −50 cm 4 s mol −1 photon −1) from the measured absorptivity ratio, δ ↑↑(15)/δ ↑↑(14) and from the measured value of 14 0 1. We also report the sum-rule-predicted hot and cold band absolute absorptivities in the other D 6h benzenes, C 6D 6, 13C 6H 6 and 13C 6D 6.

ChemInform Abstract: The S1‐S0(1B2u‐1Ag) Transition of p‐Difluorobenzene Cooled in a Supersonic Free Jet Expansion. Excitation and Dispersed Fluorescence Spectra, Vibrational Assignments, Fermi Resonances, and Forbidden Transitions.

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p‐difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been iden...

The S1–S(1B2u–1A g) transition of p-difluorobenzene cooled in a supersonic free jet expansion. Excitation and dispersed fluorescence spectra, vibrational assignments, Fermi resonances, and forbidden transitions

The vibronic spectroscopy of the S1(1B2u)–S0(1Ag) transition of p-difluorobenzene (000 at 36 838 cm−1) cooled in a supersonic free jet expansion in argon has been reinvestigated in some detail. Analysis of over 50 vibronic transitions using fluorescence excitation and dispersed single vibronic level fluorescence spectroscopy has led to the establishment or confirmation of the assignments of 19 S1 and S0 frequencies, including eight previously unassigned S1 vibrational frequencies, and the reassignment of two S1 and one S0 frequencies. Several Franck–Condon forbidden transitions have been identified. Their activity in the S1–S0 spectrum is attributed to vibronic coupling involving higher lying electronic states. Forbidden transitions involving b3g modes, notably ν27 and ν26, derive their intensity from a higher lying 1B1u electronic state, via vibronic coupling that is analogous to that responsible for the 1B2u–1Ag transition in benzene. Numerous Fermi resonances in both the S1 and S0 states have been identified. The prevalent Fermi resonance between ν′5 and 2ν6 has been analyzed with the assistance of both excitation and dispersed fluorescence spectroscopy, yielding a coupling matrix element [g566<51‖Q5‖50><60‖Q6‖62>]=−1 cm−1. Thirty-one matrix elements describing cubic anharmonicity and involving a variety of vibrational modes have been estimated. The majority of the coupling matrix elements lie within the range ±2 cm−1.

Optical absorption spectra in the quasi-two-dimensional antiferromagnets (NH3(CH2)nNH3)MnCl4(n=2, 3, 4, 5). II. Theoretical

Optical absorption spectra in the quasi-two-dimensional antiferromagnet (NH3(CH2)nNH3)MnCl4 (n=2, 3, 4, 5) were investigated. In the transitions 6A1g(S) to 4A1g(G), 4Eg(G), 4T2g(D), 4Eg(D) and 4T1g(P), much fine structure caused by electric dipole transition appeared at low temperature. It was concluded that fine structure resulted from magnon-assisted transitions by taking into account the temperature dependence of the spectra. In these compounds, when an external magnetic field was applied along the spin easy axis, a drastic energy shift in the spectra was observed at the spin-flop transition (H0=25-35 kOe). The authors reveal the origin of these spectral changes and classify the optical absorption spectra by their original excitons, taking into account these spectral changes. It is concluded that the analysis of the spectral shift at the spin-flop transition is one of the most powerful methods to assign the absorption spectra in these antiferromagnetic compounds. Spectral changes under uniaxial stress were also investigated.

Optical absorption spectra in the quasi-two-dimensional antiferromagnets (NH3(CH2)nNH3)MnCl4(n=2, 3, 4, 5). I. Experimental

Optical absorption spectra in the quasi-two-dimensional antiferromagnet (NH3(CH2)nNH3)MnCl4 (n=2, 3, 4, 5) were investigated. In the transitions 6A1g(S) to 4A1g(G), 4Eg(G), 4T2g(D), 4Eg(D) and 4T1g(P), much fine structure caused by electric dipole transition appeared at low temperature. It was concluded that fine structure resulted from magnon-assisted transitions by taking into account the temperature dependence of the spectra. In these compounds, when an external magnetic field was applied along the spin easy axis, a drastic energy shift in the spectra was observed at the spin-flop transition (H0=25-35 kOe). The authors reveal the origin of these spectral changes and classify the optical absorption spectra by their original excitons, taking into account these spectral changes. It is concluded that the analysis of the spectral shift at the spin-flop transition is one of the most powerful methods to assign the absorption spectra in these antiferromagnetic compounds. Spectral changes under uniaxial stress were also investigated.

Benzene clustered with N2, CO2, and CO: Energy levels, vibrational structure, and nucleation

Two-color time-of-flight mass spectroscopy is employed to study the van der Waals (vdW) clusters of benzene(N2)n (n≤8), benzene(CO2)n (n≤7), and benzene(CO)n (n=1,2) created in a supersonic molecular jet. Potential energy calculations of cluster geometries, normal coordinate analysis of vdW vibrational modes, and calculations of the internal rotational transitions are employed for the assignment of the benzene(solvent)1 cluster spectra in the 000 and 610 regions of the benzene 1B2u←1A1g transition. The respective vibronic and rotational selection rules for these clusters are determined based on the appropriate point groups and molecular symmetry groups of the clusters. Good agreement between the calculated and experimental spectra is obtained with regard to the vdW vibrational and internal rotational modes. The solvent molecules rotate nearly freely with respect to benzene about the benzene–solvent bond axis in the benzene(solvent)1 clusters. In the excited state a small ∼20 cm−1 barrier to rotation is encountered. Studies of larger clusters (n>2) reveal a broad red shifted single origin in the 610 spectra. A linearly increasing cluster energy shift is observed as a function of cluster size. The cluster energy shifts are not saturated by one solvent molecule on each side of the aromatic ring; several solvent molecules effectively interact with the solute π electronic cloud. Both homogeneous and inhomogeneous nucleation take place for the clusters studied depending on the ratio of the solvent–solvent binding energy to the cluster binding energy.

Two-photon excitation spectroscopy of benzene and toluene in nonpolar liquids

The two-photon excitation spectra of benzene and toluene in dilute solutions of tetramethylsilane, n-pentane, perfluoro- n-hexane, perfluoromethylcyclohexane, and perfluorodimethylcyclobutane have been measured over the laser excitation wavelength region from 360 to 560 nm, corresponding to a two-photon energy range from 55 500 to 35 700 cm −1. Vibrational analyses of the 1 B 2 u state of benzene from 36 570 to 47 280 cm −1 and of the 1 B 2 state of toluene from 36 100 to 44 700 cm −1 have been conducted and polarization measurements of the 1 B 2 u state, using both linearly and circularly polarized light, are presented and discussed. The 0-0 levels of the 1B 2u ← 1A 1g transitions of benzene and the 1B 2 ← 1A 1 transitions of toluene were observed at 38 050 ± 10 cm −1 and 37 411 ± 10 cm −1 in perfluoro- n-hexane and at 37 853 ± 20 cm −1 and 37 210 ± 20 cm −1 in n-pentane, respectively.

Infrared–ultraviolet double resonance studies of benzene molecules in a supersonic beam

We have used IR excitation to selectively create populations in admixtures of the zeroth-order states comprising the ∼3000 cm−1 ‘‘C–H stretching Fermi triad’’ of benzene. UV spectra of the 260 nm Ã(1B2u)←X̃(1A1g) transition in the IR-excited molecules show several new bands, which we have assigned. Final states in the UV transitions are some vibrational levels which have not been detected before, allowing us to find several excited-state vibrational frequencies. We have determined ν′3 =1327±3 cm−1, ν19 =1405±3 cm−1, and ν′20 =3084±5 cm−1. Also, vibrational structure which was unresolved in IR spectra of the ‘‘Fermi triad’’ was resolved in the UV double resonance spectra, confirming that the C–H stretching admixture is really a tetrad. The 3048, 3079, and 3101 cm−1 states had formerly been given the labels ν″20, ν″8+ν″19, and ν″1+ν″6+ν″19, respectively. Actually, the middle level most nearly resembles ν″1+ν″6+ν″19, and the 3101 cm−1 level is strongly mixed with ν″3+ν″6+ν″15. As predicted by molecular orbital theory, excited-state C–H bending and stretching frequencies are not very different from those in the ground state. Furthermore, we suggest that the four C–H stretching frequencies increase uniformly by ∼20 cm−1 in the excited state; reexamination of the Atkinson and Parmenter 260 nm Ã←X̃ spectrum leads us to reassign ν2 from 3130 to ∼3093 cm−1, which is 19 cm−1 above ν″2. There is a Fermi resonance between the ν6+ν′20 level and another level ∼13 cm−1 lower in energy; the strength of the perturbation is ∼18 cm−1. Possibilities for the perturbing vibrational state are ν6+ν′8+ν14 and ν′6+ν13.

Quantum-chemical investigation of Franck-Condon and Jahn-Teller activity in the electronic spectra of Buckminsterfullerene

Quantum-chemical results are reported which indicate that the absorption, fluorescence and phosphorescence spectra of Buckminsterfullerene are governed by a 1T1u ← 1Ag, 1T2g → 1Ag and 3T2g → 1Ag transition, respectively. Normal modes are calculated and their Franck-Condon and Jahn-Teller activity in these spectra are evaluated, along with the Jahn-Teller distortion of the radical anion. Vibrational progressions are expected to be short. A high phosphorescence quantum yield is predicted.

Discontinuous high spin ⇌ low spin transition in an iron(II) complex [FeL2(NCS)2] with the pharmaceutical agent bromazepam

A temperature-dependent susceptibility and Mössbauer effect study of the iron(II) complex [FeL2(NCS)2], where L is the psychotherapeutic agent bromazepam, has been performed. The Mössbauer spectra demonstrate that two lattice sites are present. One of them undergoes a high spin (5T2g)⇌ low spin (1A1g) transition around ca. 240 K, the other remains in the low-spin state over the whole temperature range. This is consistent with the temperature dependence of the effective magnetic moment µeff·, which varies between a pure low-spin value of 0.9 µB in the low-temperature region and an intermediate value of 4.2 µB as a high-temperature plateau. A small hysteresis for the high spin ⇌ low spin transition has been observed. It is suggested that two stereoisomers of [FeL2(NCS)2] with different magnetic properties are present in the lattice.

Formation d'etats triplets du biacetyle dans sa reaction en phase gazeuse avec un plasma froid d'azote

Le comportement de la molécule de biacétyle (BA) au sein du plasma froid, obtenu par extraction de l'azote soumis à une décharge électrique microonde, est étudié. Lorsque les pressions partielles d'azote et de biacétyle sont voisines (pBA ≈ 0,55 Torr, pN2 ≈ 0,60 Torr), l'émission de la transition 3Au1Ag du biacétyle est observée et présente une structure vibrationnelle. La réaction sur le biacétyle d'un plasma froid d'azote enrichi préférentiellement en N2(A3Σ+u) conduit à un renforcement d'intensité de l'émission de phosphorescence du biacétyle. L'initiateur de cette transition est l'état triplet BA(3Bu) formé par transfert isoénergétique à partir de N2(A3Σ+u) selon le mécanisme global The behaviour of the biacetyl molecule (BA) in a cold plasma, obtained by extraction of reactive species from a nitrogen plasma generated in a microwave discharge, is discussed. When the partial pressures of nitrogen and biacetyl are approximately equal (pN2 ≈ 0.60 Torr, pBA ≈ 0.55 Torr), the emission of the 3Au1Ag transition of biacetyl is observed with a vibrational structure. On reaction of biacetyl with nitrogen cold plasma preferentially enriched in N2(A3Σ+u), an enhancement of the emission intensity of the phosphorescence of biacetyl is observed. The initiator of this transition is the triplet state BA(3Bu) which originates from an isoenergetic transfer from N2(A3Σ+u) according to the overall mechanism

The UV absorption spectrum of C60 (buckminsterfullerene): A narrow band at 3860 Å

The absorption spectrum of the special C60 cluster buckminsterfullerene has been studied in a supersonic beam by laser depletion of the cold van der Waals complexes of C60 with benzene and methylene chloride. Both complexes were found to display a single, isolated absorption band in the near ultraviolet superimposed on a structureless absorption continuum. For the methylene chloride complex this feature is centered at 3860 Å, and is roughly 50 cm−1 wide. In the benzene van der Waals cluster, the corresponding feature is located at 3863 Å, and has a similar width. This spectrum is tentatively assigned to the 0–0 band of the lowest 1T1u←1Ag (LUMO+1←HOMO) transition of a truncated icosahedral carbon shell structure, broadened by coupling to the underlying quasicontinuum of ground state vibrational levels.

Two-photon spectra of the S1–S transition in glyoxal

The two-photon excitation spectrum of the 1Au(S1)–1Ag(S0) transition in trans-glyoxal is observed in the region of 21 800–25 400 cm−1. The excitation source is the H2 Raman shifted output of a pulsed dye laser. The source yields tunable output from 0.7 to 1.4 μm and enables both one- and two-color two-photon spectra to be observed. These spectra are complementary to the well known one-photon spectra for this transition. However, the two-photon selection rules cause much of the vibrational structure to be simplified. Many of the observed bands belong to the torsional mode, ν7, revealing vibrational quanta up to 12 in the excited state and 8 in the ground state. The two-photon rotational contours for a number of transitions are calculated for comparison with the observed spectra.

Doppler-free saturation spectroscopy of polyatomic molecules: Photochemical hole burning of gas phase s-tetrazine

Doppler-free spectra of two vibronic bands in the à (1B3u)←X̃ (1Ag) 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 16a11 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 (1Ag) state is the process responsible for the observed linewidths and that dissociation takes place on the electronic ground state potential surface.