The two-photon excitation spectrum of pyrene

The two-photon excitation spectrum of pyrene and the polarization ratio Ω have been measured up to ≈2100 Å. Evidence of a strong vibronic couplin     

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.     

The two-photon excitation spectrum of fluorene near 6 K

The two-photon fluorescence excitation spectrum of the fluorene single crystal at about 6 K is reported. Two k = 0 exciton bands arising from the p     

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.     

The two-photon excitation spectrum of triphenylene in n-heptane single crystals

A comparison of the one-and two-photon absorption spectra of triphenylene in n-heptane single crystals at 1.6. K reveals that the lowest singlet state (S₁) is of ¹A′₁ symmetry while the second singlet state(S₂) has ¹A′₂ symmetry, in contradiction with the commonly accepted assignment.     

The two-photon excitation spectrum of a carbazole single crystal at 4.2 K

The low-energy system in the polarized two-photon excitation spectrum of carbazole at 4.2 K has been measured by detecting fluorescence pulses from deliberately added anthracene. Herzberg—Teller activity was detected for Some A₁ vibrations. The two-photon operators contribute to the two-photon cross section in the order yy ≈ zz ? xx.     

The two-photon excitation spectrum of a dibenzofuran single crystal at 4.2 K

The polarised two-photon excitation spectrum of crystalline dibenzofuran at 4.2 K has been measured. Only totally-symmetric vibrations are active, and their relative intensities are similar to those in the one-photon spectrum already reported. No factor-group splittings are found. The origin in the two-photon spectrum has a width of ? 10 cm^-1 in contrast to a width of ? 250 cm^-1 in the one-photon spectrum, now attributed to extreme inhomogeneous broadening.     

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).     

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.     

Molecular ion fluorescence excitation spectrum from an ion beam

The laser-induced excitation spectrum for the 0.0 band of the A²II → X²X⁺ transition of CO⁺ has been observed from an ion beam. Analysis of electron-bombardment excitation fluorescence in the same ion beam indicates radiative lifetime for the A²IIν′ = 0 state of CO⁺ of 7.4 ± 1.0 μs.     

Protonation and conformational dynamics of GFP mutants by two-photon excitation fluorescence correlation spectroscopy

GFP mutants are known to display fluorescence flickering, a process that occurs in a wide time range. Because serine 65, threonine 203, glutamate 222, and histidine 148 have been indicated as key residues in determining the GFP fluorescence photodynamics, we have focused here on the role of histidine 148 and glutamate 222 by studying the fluorescence dynamics of GFPmut2 (S65A, V68L, and S72A GFP) and its H148G (Mut2G) and E222Q (Mut2Q) mutants. Two relaxation components are found in the fluorescence autocorrelation functions of GFPmut2: a 10-100 micros pH-dependent component and a 100-500 micros laser-power-dependent component. The comparison of these three mutants shows that the mutation of histidine 148 to glycine induces a 3-fold increase in the protonation rate, thereby indicating that the protonation-deprotonation of the chromophore occurs via a proton exchange with the solution mediated by the histidine 148 residue. The power-dependent but pH-independent relaxation mode, which is not affected by the E222Q and H148G mutations, is due to an excited-state process that is probably related to conformational rearrangements of the chromophore after the photoexcitation, more than to the chromophore excited-state proton transfer.     

Evolution of fluorescence resonance energy transfer between close-packed CdSeS quantum dots under two-photon excitation

Energy transfer (ET) processes between quantum dots (QDS) were investigated by means of steady-state and time-resolved up-conversion luminescence measurements. Two types of CdSeS QDs with different Se/S molar ratios at the similar sizes of ~4.5 nm emit green and orange up-conversion luminescence at infrared laser excitation, separately. The power dependence and nanosecond luminescent decays of QDs films demonstrated that up-conversion luminescence was attributed to two-photon absorption and ET process occurred from green-emitting QDs to orange-emitting QDs. The ET rate was estimated quantitatively to be 0.03 ns(-1) by Dexter theory. The decrease of ET rate is due to Se doped substituted in the Sulfur sites. The band-edge excitonic state is predominating at the initial time evolution and responsible for peak shift and ET. The surface emission of orange-emitting QDs becomes slower, and is attributed to the trapping of electrons from QDs donors.     

Level-crossing experiments by two-photon excitation

Complete orientation of the T17² P _3/2 (F=2) state by two-photon excitation allows Hanle-effect measurement in direct fluorescence as well as in the optical radiation cascade. The experiment provides estimations either of both the lifetimes of the involved excited states, or, if one of the lifetimes is reliably known, two independant determinations of the other. The measurements are reported on the background of other nonlinear Hanle-effect and level-crossing experiments.     

Reinterpretation of the fluorescence excitation spectrum of hexafluorobiacetyl in a supersonic jet

The results of the analysis of the fluorescence excitation spectrum of trans-hexafluorobiacetyl published earlier are revised. New values of torsion frequencies and potential barriers of internal rotation of the CF₃ group in the ground and excited states of the molecule are obtained. A procedure for calculating the probabilities of torsional vibronic transitions of molecules is described. Translated fromZhurnal Struktumoi Khirnii, Vol. 38, No. 2, pp. 293–302, March–April, 1997.     

Selective fluorescence recovery after bleaching of single E2GFP proteins induced by two-photon excitation.

We report the two-photon excitation and emission or a recently developed green fluorescent protein (GFP) mutant, E(2)GFP. Two main excitation bands are found at 780 and 870 nm. Blinking and irreversible and reversible bleaching were observed. Fluorescence blinking occurs in the millisecond range and has been ascribed to conversions between the neutral, anionic and dark zwitterionic states. Bleaching is observed after approximately 10 to 400 ms depending on the excitation power, and it is probably due to a conversion to a dark state. The striking feature of this GFP mutant is that the fluorescence can be recovered with very high efficiency only upon irradiation at 720 +/- 10 nm. This GFP mutant therefore seems promising as an almost permanent chromophore for two-photon excitation (TPE) microscopy or for applications in single-molecule memory arrays.     

The two-photon excitation spectrum of triphenylene in n-heptane single crystals: Chem. Phys. Letters 57 (1978) 496

Chemiluminescence spectra and photon yields resulting from reactions of copper atoms with N₂O, O₂, NO, NF₃, SF₆, F₂, Cl₂, Br₂, and I₂ have been obtained between 200 and 1100 nm. The most interesting feature of these spectra is the strong and peculiar emission from copper atoms observed in the Cu + NF₃ reaction; population inversion has been obtained between some of the Cu states and it is suggested that this is due to energy transfer from N₂(A) formed in the reaction to copper atoms.     

Two-color two-photon excitation using femtosecond laser pulses

The use of two-color two-photon (2c2p) excitation easily extends the wavelength range of Ti:sapphire lasers to the UV, widening the scope of its applications especially in biological sciences. We report observation of 2c2p excitation fluorescence of p-terphenyl (PTP), 2-methyl-5-t-butyl-p-quaterphenyl (DMQ) and tryptophan upon excitation with 400 and 800 nm wavelengths using the second harmonic and fundamental wavelength of a mode-locked Ti:sapphire femtosecond laser. This excitation is energetically equivalent to a one-photon excitation wavelength at 266 nm. The fluorescence signal is observed only when both wavelengths are spatially and temporally overlapping. Adjustment of the relative delay of the two laser pulses renders a cross correlation curve which is in good agreement with the pulse width of our laser. The fluorescence signal is linearly dependent on the intensity of each of the two colors but quadratically on the total incident illumination power of both colors. In fluorescence microscopy, the use of a combination of intense IR and low-intensity blue light as a substitute for UV light for excitation can have numerous advantages. Additionally, the effect of differently polarized excitation photons relative to each other is demonstrated. This offers information about different transition symmetries and yields deeper insight into the two-photon excitation process.     

Trivalent boron as acceptor in D-pi-A chromophores: synthesis,structure and fluorescence following single- and two-photon excitation

A series of new donor-pi-acceptor type compounds with trivalent boron as acceptor which show strong two-photon excited up-conversion fluorescence have been synthesized and one crystal structure described.     

Resolved single vibronic level fluorescence after two-photon excitation of benzene vapor: a direct confirmation of the inducing mode

Flourescence spectra have been observed from 1 and 5 torr of benzene after 504.2 nm excitation of the strongest two-photon band (linear polarization, ω₁ = ω₂) in the forbidden 260 nm ¹B_2u ← ¹A_1g absorption. The flourescence structure at 0.2 nm resolution is consistent with that expected from the fundamental ν′₁₄ in the excited state. This confirms directly the previous assignments of the dominant two-photon absorption progression as 1ⁿ_o14^L_o as well as the identity of the principal inducing mode, ν′₁₄ (b_2u). Vibrational relaxation from the level 14¹ appears similar to that observed from the one-photon levels.     

Multiphoton ionization and two-photon excitation spectroscopy of nitric oxide

The multiphoton ionization and two-photon excited fluorescence action spectra of nitric oxide are compared. At low fluence the two spectra differ when more than three photons are required for ionization. This may be due to the presence of several energy acquisition routes, providing new spectroscopic information on intermediate slates.