Two-photon absorption in the collisionless gas phase: lifetimes of new vibrational levels in benzene

Two-photon photoselection has been demonstrated for a molecule in the low pressure gas phase. The excitation has been used to populate new vibrational levels in benzene under isolated molecule conditions down to 0.1 torr. The new states display unusual behaviour as compared to the lifetimes of one-photon excited states. Most unexpectedly the lifetimes of the strongest two-photon inducing mode ν₁₄ is longer than all other lifetimes, including that of the vibrationless ¹B_2u origin. Some theoretical explanations are offered.     

Temperature dependence of the lifetime of naphthalene 1B3u under boltzmann conditions

The lifetime of naphthalene ¹B_3u was measured in the gas phase under Boltzmann conditions (argon pressures > 400 Torr) over the temperature range 368–460 K. The results were found to agree well with calculations based on an assumed energy-dependent decay constant derived from the analysis by Schlag et al. of the decay of single vibronic levels. The Boltzmann distribution over the vibronic levels of the excited state was calculated using densities of vibrational states obtained from a direct count procedure. The results support Schlag's contention that the energy dependence of the single vibronic level lifetimes in naphthalene is predominantly determined by the Franck-Condon factors; they also indicate that no additional mechanism is needed to explain the observed temperature effects.     

Moderate-resolution two-photon spectrum of the naphthalene crystal and naphthalene in durene

The two-photon absorption spectra at ～ 5 cm^?1 resolution are presented for the naphthalene crystal and naphthalene in durene at 2°K in polarized light. A rich vibronic spectrum of the ¹B_2u(3200 Å) naphthalene state is observed and some of the strongest vibronic origins are from in-plane vibrations.     

Intersystem crossing in jet-cooled naphthalene, α- and β-chloronaphthalene as studied by sensitized phosphorescence excitation spectroscopy

The fluorescence excitation and sensitized phosphorescence excitation spectra of jet-cooled naphthalene-h₈, -d₈, α-chloro naphthalene, β-chloronaphthalene have been measured. It is shown that the intersystem crossing efficiency of naphthalene is large for specific vibronic levels in S₁. This efficiency is due to an accidental resonance with the vibronic level belonging to nearby T₂ state. Cl atom accelerates the intersystem crossing rate of naphthalene by about two orders of magnitude.     

The two-photon phosphorescence excitation spectrum of triphenylene

The two-proton excited phosphorescence of triphenylene in PMMA matrix at 77 K was measured using a tunable flashlamp-pumped rhodamine dye laser in the effect spectral region 32000–36000 cm^?1. The band origin of the S₀S₂ transition, which is uncertain in the one-photon absorption spectrum, was observed at 33200 cm^?1. The vibronic features in the one-photon spectrum were reinterpreted.     

Two-photon spectroscopic behaviors and photodynamic effect on the BEL-7402 cancer cells of the new chlorophyll photosensitizer

The spectroscopic properties of a new chlorophyll derivate photosensitizer(CDP) are studied under the excitation wavelengths at 800 and 400 nm using femtosecond pulses from a Ti:sapphire laser.The damaging effect of CDP on the BEL-7402 cancer cells is also investigated upon two-photon illumination at 800 nm.The normalized fluorescence spectra of CDP in tetrahydrofuran(THF) show that two-photon and one-photon spectra have the same distributions and the same emission bands(675 nm).The life-times of two-and one-photon induced fluorescence of this molecule are of the order of 5.0 ns.By comparing the data it is shown that there is some difference between the two lifetimes,but the differ-ence is less than one nanosecond.The two-photon absorption cross section of the molecule is also measured at 800 nm and estimated as about σ′2 ≈ 31.5×10-50 cm4·s·photon-1.The results of two-photon photodynamic therapy(TPPDT) tests show that CDP can kill all of the tested cancer cells according to the usual Eosine assessment.Our results indicate that the two-photon-induced photophysical,photo-chemical and photosensitizing processes of CDP may be basically similar to those of one-photon ex-citation.These behaviors of the sample suggest that one may find other possible methods to estimate some photosensitizers' effects in details such as their distribution in cells and the reactive targets of the sub-cellular parts of some tumor cells via two-photon excitation techniques.     

Picosecond fluorescence decays from vibrational levels in the S1(n,π*) state of pyridine vapor

Fluorescence lifetimes of pyridine vapor were measured by exciting at various vibrational bands in the lowest-energy region of the S₁(n,π^*) ← S₀ transition. The lifetime varies between 35 and 60 ps, depending on the vibronic level excited. The non-radiative decay from S₁ is characterized by particularly fast S₁ → S₀ internal conversion.     

The two-photon spectrum of liquid pyridine by thermal lensing techniques

Thermal lensing is exploited to investigate the electronic properties of pyridine. The two-photon spectrum and polarization ratio of liquid pyridine has been measured from 2900 to 2000 Å. Absorptions corresponding to the two lowest ππ* states, of benzene parentage, and to the second excited nπ* state have been assigned. The latter, of A₂ symmetry, borrows two-photon intensity from A₁ states through vibronic coupling induced by a₂ vibrations. Calculations of allowed contributions to the two-photon cross sections for the lowest four excited levels support the assignment.     

Excited state dynamics and spectroscopy of the 1A″ state of NSF vapor — comparisons with SO2

The fluorescence emission spectrum and analysis of NSF vapor is presented. Single vibronic level excitation near the S₁ origin gives rise to a 10 μs radiative decay. The fluorescence lifetime for excitation of levels with ? 4500 cm^?1 excess vibrational energy becomes controlled by a unimolecular radiationless process which is likely photodissociation; the dependence of this radiationless rate on energy and vibrational mode is investigated. The perturbations resulting from coupling of zero-order S₁ states with other vibronic levels which control the excited state dynamics of SO₂ are apparently not operative for NSF. Attempts are made to rationalize the grossly different dynamic behavior of the S₁ levels of these two otherwise very similar systems.     

Fluorescence of derivatives of all-trans-1,6-diphenyl-1,3,5-hexatriene

Fluorescence quantum yields and lifetimes have been measured for ring-substituted derivatives of all-trans-1,6-diphenyl-1,3,5-hexatnene in several non-polar solvents over a range of temperature. The fluorescence quantum yields were high in all non-polar solvents and showed small decreases with increasing temperature. The fluorescence lifetimes varied with the nature of the substituent group and showed a marked solvent dependence. The results are interpreted in terms of a model in which thermal repopulation of S₂ from S₁ proceeds at a rate which is comparable to radiative and non-radiative decay from S₁. It is shown that neglect of Franck-Condon packing strain effects leads to overestimates of the true S₂-S₁ energy differences. Significant S₂-S₀ emission can interfere with attempts to quantify enhancement of the S₁-S₀ transition by intensity borrowing from the S₂-S₀ transition, leading to serious overestimates of vibronic coupling matrix elements.     

Radiative and non-radiative processes in jet-cooled NCNO

Lifetimes of excited vibronic levels in NCNO are measured both by LIF and by monitoring excited state absorptions. Fluorescence lifetimes are longer than S₁ radiative lifetimes at all wavelengths between the band origin (11339 cm⁻¹) and D_o (17085 cm⁻¹). In the language of radiationless transitions, the behavior below D_o is characteristic of the “small or intermediate molecule lim     

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.     

Some comments on vibronic intensities of naphthalene fluorescence spectra from single vibronic levels

It is shown that some features of intensity distribution among certain vibronic transitions in naphthalene molecule can be understood, when one takes into account adiabatic and nonadiabatic interaction between S₁(¹B_3u), S₂(^tB_2u), and S₃(^IB_3u) electronic states. the vibronic activity of the $\text{6}\text{?}$(b_1g) mode in naphthalene-d₈ can be explained in terms of an anharmonic coupling with the $\text{7}\text{?}$(b_1g) mode. The theoretical analysis suggests reinterpretation of some vibronic transitions.     

Two-photon spectroscopy in the gas phase: first excited state of naphthalene, 1B3u

Molecular two-photon spectroscopy is promising to be a new tool for finding otherwise forbidden spectroscopic transitions. Work in the gas phase is particularly important for a study of vibronic two-photon states. The proof of such a new method, at least in part, must be based on its ability to make new assignments. Benzene has up to now been the first, and only example for new assignments. We now present naphthalene as a second system for this new technique. Assignments of new states are presented and based on a spectrum at a pressure of only 70 mtorr. The analysis makes use of rotational structure, polarization behavior and hot band information. We assign not only six new fundamentals in the first excited singlet state ¹B_3u but also fix some ground state assignments, which were ambiguous in the IR spectrum.     

Vibronic interactions in s-triazine spectra

The two-photon absorption spectrum of s-triazine has been measured using multiphoton ionization. A vibronic coupling scheme, comprising the first excited ¹E″ state coupled by the ν₆(e′) vibration to the ¹A₂″ and ¹A₁″ states, successfully reproduces the essential features of the absorption, fluorescence and two-photon absorption spectra.     

Vibronic coupling calculation of the pre-resonance Raman intensities of naphthalene

The relative enhancement of the Raman intensities in the pre-resonance region has been computed for naphthalene, using the vibronic coupling expansion proposed by Albrecht. The relevant vibronic and transition moments have been computed in the CNDO/S approximation, while the normal coordinates of naphthalene have been evaluated using a MINDO/3 program. The experimental pattern of intensities is reproduced in a satisfactory way both for b_3g and a_g vibrations. The most important states and the main vibronic terms in the expansion are individuated and discussed with respect to the vibronic borrowing present in the Fluorescence spectrum of naphthalene.     

Electronic and vibronic contributions to two-photon absorption in donor-acceptor-donor squaraine chromophores

Many squaraines have been observed to exhibit two-photon absorption at transition energies close to those of the lowest energy one-photon electronic transitions. Here, the electronic and vibronic contributions to these low-energy two-photon absorptions are elucidated by performing correlated quantum-chemical calculations on model chromophores that differ in their terminal donor groups (diarylaminothienyl, indolenylidenemethyl, dimethylaminopolyenyl, or 4-(dimethylamino)phenylpolyenyl). For squaraines with diarylaminothienyl and dimethylaminopolyenyl donors and for the longer examples of 4-(dimethylamino)phenylpolyenyl donors, the calculated energies of the lowest two-photon active states approach those of the lowest energy one-photon active (1B(u)) states. This is consistent with the existence of purely electronic channels for low-energy two-photon absorption (TPA) in these types of chromophores. On the other hand, for all squaraines containing indolinylidenemethyl donors, the calculations indicate that there are no low-lying electronic states of appropriate symmetry for TPA. Actually, we find that the lowest energy TPA transitions can be explained through coupling of the one-photon absorption (OPA) active 1B(u) state with b(u) vibrational modes. Through implementation of Herzberg-Teller theory, we are able to identify the vibrational modes responsible for the low-energy TPA peak and to reproduce, at least qualitatively, the experimental TPA spectra of several squaraines of this type.     

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance-Enhanced-Multiphoton-Ionization and Fourier Transform VUV Absorption Spectroscopy

The VUV absorption spectrum of fenchone is re-examined using synchrotron radiation Fourier transform spectrometry, revealing new vibrational structure. Picosecond laser (2+1) resonance enhanced multiphoton ionization (REMPI) spectroscopy complements this, providing an alternative view of the 3spd Rydberg excitation region. These spectra display broadly similar appearance, with minor differences that are largely explained by referring to calculated one- and two-photon electronic excitation cross-sections. Both show good agreement with Franck-Condon simulations of the relevant vibrational structures. Parent ion REMPI ionization yields with both femtosecond and picosecond excitation laser pulses are studied as a function of laser polarization and intensity, the latter providing insight into the relative two-photon excitation and one-photon ionization rates. The experimental circular-linear dichroism observed in the parent ion yields varies strongly between the 3s and 3p Rydberg states, in good overall agreement with the calculated two-photon excitation circular-linear dichroism, while corroborating other evidence that the 3p_z sub-state plays no more than a very minor role in the (2+1) REMPI spectrum. Vibrationally resolved photoelectron spectra are recorded with picosecond pulse duration (2+1) REMPI at selected intermediate vibrational excitations. The 3s intermediate state displays a very strong Δv=0 propensity on ionization, but the 3p intermediate evidences more complex vibronic dynamics, and we infer some 3p→3s internal conversion prior to ionization.     

A study of the two-photon spectrum of and vibronic coupling in the first system of pyrene

The polarized, low-energy, two-photon absorption system of pyrene-h₁₀ and pyrene-d₁₀ in fluorene and biphenyl host crystals at 4.2 K has been measured and analyzed to provide firm symmetry assignments. Good agreement was found for the energies of the two-photon band positions in a heptane matrix recorded using a site-selective technique. The transition has A_g vibronic symmetry being induced by single quanta of several b_2u fundamentals; very short progressions in some a_g modes were observed. Vibronic coupling involving a_g modes in the one-photon spectra has been examined. Relative line intensities have been measured for seven fundamentals in absorption and fluorescence together with some overtones and binary combinations. First-order Herzberg—Teller theory can account for these intensities and the intensities in the two-photon spectrum. The vibronic coupling is of medium strength, and plays an important role in determining the intensities of the bands in the spectrum because the allowed component of the transition is weak. In the one-photon, the strength of the coupling is weaker in a biphenyl than in a heptane matrix; in the former case, it is assumed that there is a transition dipole interaction that acts to transfer intensity from the guest to the host in such a way as to reduce the transition moment to the pyrene “lending” state involved in the vibronic coupling.     

Illuminating the origins of two-photon absorption properties in fluorescent protein chromophores

We provide here a structural impact on two-photon absorption cross-section (σ^TPA) for 22 distinct fluorescent protein (FP) chromophores. By employing time-dependent density functional theory, we gain insight into two-photon absorption (TPA) process by investigating relationship between σ^TPA and one-photon electronic transition dipole moment and permanent dipole moment change (Δμ) upon transition. Our results reveal that for the S₁ excited state, σ^TPA is proportional to (Δμ)² in agreement with two-state model of TPA process. On the contrary, the TPA spectroscopy of higher excited states (S _n, n > 1) is much more complex. We do not find a main driving force of large σ^TPA that would be common for investigated chromophores. Instead, it seems that channel interference between one-photon transition dipole moment vectors is responsible for enhancement or diminishment of σ^TPA. Our in vacuo results may serve as a benchmark to investigate a role of chromophore-protein interaction in shaping TPA spectra of FPs.