Two-photon absorption study of Zn(II)tetraphenylporphin: Role of a higher excited singlet state of gerade symmetry

The S₂ state fluorescence of Zn(II)tetraphenylporphin has been studied by using two-photon absorption and optical—optical double-resonance techniques. The main process to populate the S₂ state was found to be a stepwise two-photon absorption to the S_nstate through the S₁ state. The large absorption cross section of the S_n ← S₁ transition (6.8 × 10^?16 cm² molecule^?1) at 540 nm suggests that there exists a higher excited singlet state of gerade parity.     

Sn ← S1 and Tn ← T1 absorption spectra of highly purified chrysene in solution

Time-resolved S_n ← S₁ and T_n ← T₁ absorption spectra were observed for highly purified chrysene in THF solution. Formerly assigned to the S₄ ← S₁ band located in the 17200–17600 cm⁻¹ (581-568 nm) region. S_n ← S₁ was reassigned to S₆ ← S₁. The S₄ ← S¹, S₅ ← S₁, S₇ ← S¹ and S₈ ← S₁ bands were also observed at 13500 cm⁻¹ (740 nm). 15700 cm⁻¹ (635 nm). 19000 cm⁻¹ (525 nm), and 20400 cm⁻¹ (490 nm), respectively. The relevant molar extinction coefficients were 7100 (S₄ ← S₁), 15000 (S₅ ← S¹), 14000 (S₆ ← S₁), 19000 (S₇ ← S₁), and 14000 M⁻¹ cm⁻¹ (S₈← S₁).     

Effect of rotational relaxation on the intensity and polarization of fluorescence emission caused by sequential two-photo excitation

General formulas are derived for the intensity and the degree of polarization of the S_m-fluorescence emission (m ? 2) of a sample excited by the sequential two-photon excitation process (S_n ← S₁ ← S₀, n ? 2) with plane-polarized pulsed light. They show how the S_m-fluorescence intensity and anisotropy depend on the relative orientation of the relevant transition dipoles within a molecule and on the degree of rotational relaxation of molecules in the intermediate state (S₁) and in the S_m state (for the case m = n), or in the S_n → S_m process (m ≠ n).     

Resonant two-photon ionization spectra of o,p, m-xylene...Ar n (n = 1, 2)

The resonant two-photon ionization technique (R2PI) is used to study jet-cooled van der Waals (vdW) complexes o, p, m-xylene...Ar _n (n = 1, 2) through the S ₁ ← S ₀ transition around the origin band. We have tentatively assigned the main spectral features of these vdW complexes. The transitions of these complexes are red-shifted from those of the monomers. The influence of the substituent positions on the vdW vibrations is investigated.     

Theoretical study on geometry and spectroscopic properties of 1,1′-binaphthyl in the electronic ground and first excited singlet states

Equilibrium geometries for the electronic ground and first excited singlet states of 1,1'-binaphthyl have been calculated by minimization of the total energy with respect to all internal coordinates. Using these results, an interpretation of the fluorescence S₁→ S₀ and absorption spectra S_m ← S₀ and S_n ← S₁ in rigid and fluid solutions is given.For the first time the equilibrium geometry of the first excited singlet state of 1, 1′-binaphthyl has been calculated. On excitation to the S₁ state the dihedral angle θ between the two naphthalene moieties is de- creased from 61 ° to 41 °. A detailed survey of CH bond lengths in the S₀ and S₁ states has been given. This result should be of particular importance for the theoretical treatment of radiationless transitions.Using equilibrium geometries for the S₀ and S₁ states a satisfactory interpretation of the S_m ← S₀ and S_n ← S₁ absorption spectra as well as of the fluorescence spectra in fluid and rigid solutions can be given. Concerning the S_n ← S₁ absorption spectrum in fluid solution, the calculations predict a strong absorption (A ← B transition) in the still uninvestigated region of energies lower than 11000 cm^?1.From the results of this paper and of other calculations it can be concluded that the Warshel-Karplus method yields reliable equilibrium geometries for electronic ground and excited states of unsaturated hydrocarbons [22,23].     

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.     

Two-photon (A1B2 ← X1 A1) supersonic jet spectrum of aniline

The supersonic jet multiphoton ionization (2 photons to resonance, 4 to ionization) ¹A ← ¹X spectrum of aniline is reported in the 560–590 nm region. The two-photon ¹A ← ¹X spectrum is very similar to the analogous one-photon spectrum. In particular, ν₁₄, the Kekulé signature mode of two-photon ¹L_b substituted benzene spectra is not prominent, but two quanta of the amino inversion mode, ν_I, are. A dispartiy between theoretical calculations of the ¹A ← ¹X two-photon cross-section, and the thermal lensing cross-section reported by Rice and Anderson [J. phys. Chem. 90, 6793 (1986)] is noted. The theoretical considerations only account for about half the thermal lensing intensity.     

Two-photon spectra of gases by three-wave mixing

We show that the two-photon resonances in the third-order susceptibility can be exploited to yield two-photon spectra of molecular gases at moderately high spectral resolution. This form of spectroscopy does not depend on the occurrence of processes (such as fluorescence of photoionization) leading to indirect methods of two-photon absorption. The method is direct and leads in principle to values for a two-photon cross section. Comparisons of two-photon and coherent anti-Stokes Raman resonances leads to ratios of Raman and two-proton cross sections independent of the laser powers and spatial characteristics. The technique is documented with rotationally resolved spectra of SO₂ and NO. A value for |α_xxα_yy| of 1.5 × 10^?52 cm⁶ was measured for the O₁₂ (6$\text{built:1}\text{2}$) component of the (A)²Σ ← (X)² Π_{$\text{1}\text{2}$} transition of nitric oxide.     

Cars study of vibrationally excited H2 produced in formaldehyde dissociation

Coherent anti-Stokes Raman scattering (CARS) has been used to observe H₂ from H₂CO + hv H₂ + CO. Photolysis of K = 1 (ortho) lines of the 2¹4¹ S₁ ← S₀ transition produced no para-H₂ (<4% statistical); and demonstrated nuclear spin conservation. H₂ states v = 1, 2 and 3 were comparably populated.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. II. Calculation of spin-orbit coupling

A calculation of the spin-orbit coupling in the lowest excited triplet state of tetramethyl-1,3-cyclobutanedithione (TMCBDT) has been performed. The results show the following. (1) In the TMCBDT crystal the ground singlet-to-lowest triplet transition moment is predicted to be exclusively ? c polarized, as observed. (2) The assignment of the lowest triplet state should be ³A_u as found earlier for the oxygen analog. TMCBD. (3) The two largest contributions (～ 60%) to the isolated-molecule T₁ → S₀ transition moment come from the two triplet-triplet transitions.

and

, both of which are polarized along the CS bonds. (4) The total contribution to the transition moment parallel to the CS bonds is 76% from the T_l ← T₁ transitions and 24% from the S_γ ← S₀ transitions. And, finally, (5) the calculated oscillator strength of 2 × 10^?4 for the largest T₁ ← S₀ component (along y) falls within the range of typical spin-allowed, singlet-singlet n-⁼ transitions.     

Two-photon laser spectroscopy of the muonium 1S–2S transition

The 1² S _1/2(F=1)?2² S _1/2(F=1) energy interval in muonium has been investigated by Doppler-free two-photon laser spectroscopy. The transition was observed by uniquely identifying and counting the muons released after the photoionization of the 2S state by the same laser field. The measured transition frequency of 2455 528016(58)(43) MHz is in good agreement with QED calculations. The experiment can be interpreted as a test of the Lamb shift contributions at the 1% level. The method is also well suited for a precise determination of the mass of the positive muon in a muoniumhydrogen isotope shift measurement.     

The production of Te atoms by the multiphoton dissociation of diethyltelluride

Detection of Te atoms in the ground ³P^2,1,0 states has been accomplished using two-photon laser-induced fluorescence (LIF) and “2 + 1” multiphoton ionization (MPI). Te atoms were produced by multiphoton dissociation of (C₂H₅)₂Te in the region 358–395 nm. The LIF and MPI experiments utilize selective excitation of the 6p ³P_J′←5p ³P_J″ and 7p ³P_J′←5p ³P₂ transitions by two-photon absorption. Line strength intensities for the individual J′ ← J″ fine structure pairs of the 6p ³P_J′ ← 5p ³P_J″ two-photon transitions were calculated and compared to the LIF data. Intensities of these transitions of Te atoms are compared to analogous ³P ← ³P transitions in S and O atoms.     

Sub-doppler two-photon spectrum of asymmetric rotor molecules: analysis of the qqQ rotational branch of the S1(1B2u)141 ← S0(1A1g) system of benzene-d1

The Doppler-free two-photon excitation spectrum of the ^qqQ branch of the 14¹₀ vibrational band of the S₁(¹B_2u) ← S₀(₁A_1g) transition of benzene-d₁ has been recorded using a cw single-mode dye laser coupled to an external concentric resonator. The spectrum has been analysed using a non-rigid Watson Hamiltonian. More than 200 lines with J up to 20 have been assigned and the rotational constants which best reproduce the spectrum are A¹_v = 0.181435, B¹_v = 0.169990, C¹_v = 0.089055 cm⁻¹. The K_a = odd lines of the ^qqQ₅(J) subbranch show small and quite regular perturbations of 60 ± 5 MHz which are probably due to a coupling to another vibrational state of the S₁ manifold.     

Optical homogeneous linewidths of resorufin in ethanol glass: An apparent contradiction between hole-burning and photon-echo results?

Spectral holes have been burnt in the S₁ ← S₀ 0-0 transition of resorufin in two phases of ethanol glass between 0.3 and 4.2 K. The holewidths differ by a factor of 8 and follow a T^1.3 dependence. The homogeneous linewidths (Γ_hom) in one of the phases are about halfthose derived from a photon-echo experiment by Fayer and co-workers. Holes burnt between ≈600 and 573 nm yield the same holewidth. It is concluded that hole-burning measures Γ_hom in this amorphous system.     

Measurement of kinetic processes in photoexcited stilbene solutions

The kinetics of photophysical processes has been measured in stilbene solutions in the temperature range between ?40°C and 20°C. The population of the S₁ level excited by two-photon absorption (TPA) and of secondary populated levels has been investigated using a probe beam method. It was found an energy barrier ΔE = 5 × 10² cm^?1 of the thermically activated transition S₁ → 1′. The rate parameter of this transition is determined to be k^?1_11′, ≈ 2 ps at very high temperatures.     

Correlation of singlet energies of aromatic hydrocarbons with the rates of protonation of their anion radicals

Formation and protonation of aromatic anion radicals in 2-propanol were studied by kinetic spectrophotometric pulse radiolysis. All polycyclic hydrocarbons studied were found to react very rapidly with e^?_solv. Those with relatively high electron affinity were also reduced by (CH₃)₂CO^?. The anion radicals formed undergo protonation by direct reaction with the alcohol molecule. The rate constants for this protonation vary from ≈ 6 × 10⁵ s^?1 for cis-stilbene and naphthalene down to 20 s^?1 for perylene. The variations in rates are discussed in terms of changes in singlet energy separation (ΔE_{S1 ← S0}). The logarithm of the protonation rate constant for alternant hydrocarbons is linearly dependent on ΔE_{S1 ← S0}.     

π* ← n electronic absorption spectrum of 2,6-dichloropyrazine

The π* ← n electronic absorption system of 2,6-dichloropyrazine, corresponding to the ¹B_3u ← ¹A_1g transition of pyrazine, has been recorded in the vapour phase and in solution in cyclohexane. A vibrational analysis of this system has been proposed and it is shown that vibronic interaction between two excited states of 2,6-dichloropyrazine exists. Another system is observed in the solution spectrum of this molecule in cyclohexane and it is shown to be a π* ← π transition analogous to the ¹B_2u ← ¹A_1g transition in pyrazine.     

Population hole-burning using a triplet reservoir: S1 ← S0 transition of zinc porphin in n-octane

Optical hole-burning has been observed in the S₁ ← S₀ absorption of zinc porphin in n-octane by selective depletion of the ground state population and storage in the mestastable triplet state. In this way the homogeneous linewidths of S₁ ← S₀ for molecules in A and B sites of n-octane were measured between 1.6 and 4.5 K. Thermally induced dephasing was observed for B-site molecules with an activation energy of 14 cm^?, equal to the separation between the two electronic components of S₁.     

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.     

Excited-state absorption spectroscopy and state ordering in polyenes: 1,3,5,7-Octatetraene

The S_n ← S₁ spectrum of 1,3,5,7-octatetraene has been obtained in cyclohexane. Calculations predict different S_n ← S₁ spectra for the lowest excited ¹Ag^? or ¹B_u⁺ states. The experimental S_n ← S₁ spectrum is consistent with the 2 ¹A_g^? as the lowest excited state. Extension of this technique to smaller polyenes is discussed.