The polarised two-photon excitation spectrum of benzene monocrystals at 6 eV

The two-photon excitation spectrum of a benzene single crystal at 4.2 K has been recorded in the region of the second absorption system. The onset of two-photon absorption occurs near 46 500 cm^?1 (quoted as a two-photon frequency). The spectrum has the appearance of a forbidden transition in that the system begins with weak lines which become dominated by an intense continuum at higher energies. The two-photon cross section at 55 000 cm^?1 (the limit reached in this study) is about 200 times greater than at 47 490 cm^?1 although the peak of this strongly allowed system has not yet been reached. The fwhm of the bands near 47 000 cm^?1 is 280 cm^?1, the same as in the one-photon spectrum at these energies. The polarisation ratio is much the same over the entire energy range, and is consistent with the two-photon operators (xz, yz) or (zz). An analysis of all the data available from the one- and two-photon spectra suggests that the transition is ¹B_1u ← ¹A_1g in which the vibronic intensity is derived from the ¹E_1u state in the one-photon and ¹E_1g in the two-photon spectrum.     

Vibronic coupling of pyrene in the first ππ* excited state

The two-photon fluorescence excitation spectrum of pyrene in n-hexane and n-heptane matrices has been measured at 10 K in the region of the first electronic transition (26800–30200 cm^?1). The spectrum consists of a rich number of sharp bands, being in general better resolved in n-hexane than in n-heptane matrix. Shpol'skii multiplets have been observed for the most intense bands. A strong two-photon band dominates the spectrum = 1495 cm^?1 from the 0—0 line and was assigned to B_1u × b_1u = A_g symmetry. Other weaker vibronic origins occur in the spectrum which were correlated to vibrational modes of b_1u, b_2u, b_3u and a_u symmetry. Intense vibronic bands are observed close to the origin of the second electronic transition and were interpreted as combination bands of B_1u × b_1u × b_3g symmetry. A two-photon vibronic theory to account for their intensity is proposed where the electronic moment is linearly expanded in powers of the nuclear displacements.     

Two-photon absorption spectrum of (2,2)-paracyclophane

The two-photon absorption spectrum of crystalline (2,2)-paracyclophane was measured in the regions of 31200–33800 cm^?1, 34700–36500 cm^?1, and 40800–48600 cm^?1 by monitoring the fluorescence intensity using a tunable dye laser as a two-photon excitation source. Two absorption bands in the region 34700–36500 cm^?1 were assigned to the two even-parity allowed ¹B_1g ← ¹A_1g and ¹B_2g ← ¹A_1g transitions.     

Two-photon excitation of SF6

We calculate the excited-state populations and index of refraction in the two-photon excitation of SF₆ from ν₃ = 0 to the A_1g sublevel of ν₃ = 2 for a thermal distribution of initial rotational states at temperatures of 30 and 140 K.     

Non-linear infrared spectroscopy of SF6 at 30 K

Calculations of non-linear absorption by SF₆ at 30 K from a tunable infrared laser are presented and compared with a recent experiment of Alimpiev and co-worker. Theory and experiment agree qualitatively and there is reasonable quantitative agreement on the positions and strengths of two-photon Q-branch reasonances which excite the 2 v₃ A_1g and E_g substates.     

The two-photon spectrum of toluene vapor

The two-photon excitation spectrum of toluene-h₈ and toluene-d₈ vapor has been recorded under low resolution (1 cm^?1) in the region of the S₁ ← S₀ (¹B₂ ← ¹A₁) transition. Although the electronic transition is formally allowed in two-photon spectroscopy, a large fraction of intensity exists in a subsystem induced by the out-of-phase CC stretching vibration ν₁₄ (b₂). Band contours associated with each of the two assigned tensor components of the transition are identified and partially analyzed by comparison with the two-photon contours of fluorobenzene.     

The lower valence states of linear polyenes: Locations and identifications

The experimental transition energy to the valence A_g state of butadiene is used to validate a subset of the theoretical calculations that have been carried out on this molecule. The validated calculated A_g transition energies of these calculations are then compared with the experimental B_u transition energies to determine the relative state ordering in isolated, unsubstituted butadiene, hexatriene, and octatetraene. The A_g origins are concluded to lie below the B_u origins for all polyenes except butadiene. Experimental confirmation of this conclusion is presented. The extreme breadth of the A_g transition, 1.0 eV, is noted. Some possible implications of this breadth are discussed.     

Two-photon absorption of liquid pyridine: a study using thermal lensing and cars spectroscopy.

The two-photon spectrum of liquid pyridine has been obtained by thermal lensing and CARS bandshape analysis. Four transitions have been detected between 315 and 200 nm. The maximum occurring at 244 nm has been assigned to the two-photon allowed n>π* transition of A₂ symmetry.     

Herzberg-teller effect and intensity distribution in the absorption and raman scattering resonance spectra of polyatomic molecules

This paper studies the influence of the Herzberg-Teller effect on the intensity distribution in one- and two-photon absorption and Raman scattering resonance spectra of substituted benzenes. A quantum mechanical analysis of the intensity distribution in the spectra of phenylacetylene and pyrazine is accomplished using the general approach suggested in [1, 2]. Due to the inclusion of vibronic coupling in the calculation, one can explain the presence of lines (as well as their combinations and overtones) arising from one-quantum excitation of nontotally symmetric vibrations. The calculated data are in good agreement with the experimental data. Significantly, the two-photon absorption spectrum of pyrazine in the region of the¹A_1g→¹B_3u long-wave singlet-singlet transition is explained only by the Herzberg-Teller effect. Saratov State Pedagogical Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 304–309, March–April, 1995. Translated by L. Smolina     

Two-photon spectroscopy in the gas phase: 1B2u ← 1Ag transition of p-difluorobenzene

The two-photon excitation spectrum ¹B_2u ← ¹A_g of p-difluorobenzene in the gas phase is presented and analysed. The normal absorption is electric dipole allowed and shows no vibronic coupling, but the two-photon absorption is electric dipole forbidden and displays rich vibronic structure. Eight vibronic origins are assigned to their excited state fundamentals by analysis of the hot bands and by analogy with benzene. The only previously unassigned ground state frequency, an a_u mode, is active in the spectrum and is accordingly assigned. The sequences and the abundant Fermi resonances accompanying absorption are also partially assigned.     

Spectral band assignments for Cu+ in alkali halides via two-color two-photon spectroscopy

A strong beam + weak beam two-color two-photon spectroscopy was used to obtain spectra from the interconfiguration transition d¹⁰ → d⁹s of Cu⁺ impurity in some alkali halide hosts. The selection rules for two counterpropagating beams independent choice of polarizations to obtain pure E_g and T_2g bands are also given. The results show better agreement with theoretical calculations than that front single beam arrangement.     

Magnetic circular dichroism of the 3A2g → 3T2g band of MgO: Ni2+

The absorption and magnetic circular dichroism (MCD) spectra of the ³A_2g → ³T_2g transition in Ni²⁺:MgO have been studied. MCD data confirm the magnetic dipole character of the zero-phonon lines. The phonon side bands can be explained in terms of an A_1g lattice mode.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

The excited electronic states of all-trans-1,3,5-hexatriene

Extensive configuration interaction calculations based on ab initio wavefunctions including diffuse basis functions are reported for all-trans-1,3,5-hexatriene. Using these results we have assigned the one-photon spectra of Gavin and Rice and the electron-impact spectra of Kuppermann, and we have confirmed the assignment of the two-photon spectra of El-Sayed. The valence 2 ¹A_g state is found to lie above the strongly allowed valence 1 ¹B_u state.     

Light absorption by manganese pairs in KMgF3:Mn2+ through a vibronically induced exchange mechanism

Double excitations to ⁴E_g and ⁴A_1g states in manganese pairs of KMgF₃:Mn²⁺ have been studied spectroscopically. Three prominent sharp bands are observed in the low temperature absorption spectrum. These bands are displaced by about 400–500 cm^?1 to higher energies from the expected electronic origins. It is proposed that the observed pair transitions gain their intensity through a vibronically induced exchange mechanism. With this mechanism the symmetric double excitations ⁶A_1g⁶A_1g → ⁴E_gu⁴E_gu, ⁴E_gv⁴E_gv and ⁴A_1g⁴A_1g become allowed in addition to the transition ⁶A_1g⁶A_1g → ⁴E_gu⁴A_1g. Analogy to the spectrum of the linear dinuclear chromium complex [(NH₃)₅CrOCr(NH₃)₅]⁴⁺, where the same mechanism has been postulated, is demonstrated.     

Polarization and assignment of the two-photon excitation spectrum of benzene vapor

The two-photon excitation (TPE) of benzene fluorescence in the vapor phase at 60 torr is reported for the total-energy region from 38 086 cm^?1 to 42 441 cm^?1 using both circular and linear polarized light from a nitrogen-pumped dye-laser. The theory of the polarization dependence of the vibronic transitions in benzene is briefly reviewed, and it is seen how transitions involving vibrations of b_1u symmetry are expressly forbidden for this type of TPE experiment in which the two photons are identical. Five vibronic origins with distinctive rotational contours and polarization dependence are identified in the TPE spectrum. The υ₁₄(b_2u) vibronic origin at 1570 cm^?1 (above the electronic origin of the ^IB_2u state) stands out very prominently in the linear polarized spectrum, but nearly disappears in the circular polarized spectrum. This striking polarization dependence indicates a significant contribution of A_2u electronic states to the intermediate states of this TPE vibronic transition. The relatively great strength of the υ₁₄ band may be due to vibronic borrowing by the b_2u mode from the ground electronic state (A_1g).     

Assignment of the vibrational bands of the B1u ← A1g transition in solid benzene films

Low temperature absorption spectra of benzene films were observed in the ¹B_1u ← ¹A_1g transition region. The origins of the two progressions of the totally symmetric vibration v₂(a_1g) are assigned to the crystal-field-induced 0—0 transition and to the false origin 0 + v_1g(e_2g).     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

Light absorption of CrIII in two host lattices of pyroxene and spinel type

The light absorption of Cr³⁺ – doped diopside CaMgSi₂O₆ and spinel ZnGaAlO₄ was measured spectrophotometrically. Two absorption bands corresponding to the electron transition: ⁴A_2g → ⁴T_2g (low frequency band) and ⁴A_2g → ⁴T_1g (high frequency band) were obtained. Generally, the value of the estimated ligand field parameters Δ and B showed that the Cr? O bond in the studied two structures is of different character.