Time-resolved Fluorescence Depletion Research Articles

Optimal Dynamic Discrimination in Tryptophan-Containing Dipeptides

Optimal Dynamic Discrimination based on the phase-shaping of deep ultraviolet femtosecond pulses was applied to selectively modulate the time-resolved fluorescence depletion of pairs of tryptophan-containing dipeptides. Our results indicate that phase-sensitive excitation allows their differential identification, beyond the limits of linear and time-resolved spectroscopy.

Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50 fs laser pulse at 400 nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S5 of LD 700.

Time-resolved rotational spectroscopy of para-difluorobenzene·Ar

We report on time-resolved rotational spectroscopy experiments of the cluster para-difluorobenzene·Ar ( pDFB·Ar) by picosecond laser pulses in a supersonic expansion. Rotational coherences of pDFB·Ar are generated by resonant electronic excitation and probed by time-resolved fluorescence depletion spectroscopy and time-resolved photoionization ((1+1′) PPI) spectroscopy. The former allows the determination of both ground and excited state rotational constants, whereas the latter technique enables the separate study of the excited state with the benefit of mass-selective detection. Since pDFB·Ar represents a near symmetric oblate rotor, persistent J-type transients with t J ≈ n/2( A+ B) could be measured. From their analysis, (A″+B″)=2234.9±2 MHz and (A′+B′)=2237.9±2 MHz were obtained. A structural investigation, based on data of the pDFB monomer, is presented resulting in a pDFB·Ar center-of-mass distance of both moieties of R z=3.543±0.017 A ̊ with a change of ΔR z=−0.057±0.009 A ̊ upon electronic excitation. These results are compared to data of former frequency-resolved experiments and ab initio computations.

High-Resolution Rotational Coherence Spectroscopy of the Phenol Dimer

We report on an investigation of the phenol dimer by high-resolution rotational coherence spectroscopy (RCS) using the method of time-resolved fluorescence depletion (TRFD). With this technique we determined with high precision the rotational constants of the ground and electronically excited states. The phenol dimer is an ideal model system to study aromatic−aromatic interaction under the constraints of an intermolecular hydrogen bond, which leads to its unique “V-shaped” structure. The TRFD investigation was complemented by an (1 + 1‘) pump−probe ionization (PPI) experiment in order to unequivocally assign ground and excited-state transients. Seven different types of RCS transients have been observed in the RCS spectrum and assigned to H‘ ‘-, H‘-, J‘ ‘-, J‘-, C-, K-, and A-type transients. From a detailed analysis by a grid search procedure based on numerical simulations of RCS spectra and a nonlinear least-squares fitting routine, the following values for the rotational constants have been obtained: A‘ ‘ = 1414.4 ± 0.6 MHz, B‘ ‘ = 313.7 ± 0.8 MHz, C‘ ‘ = 287.5 ± 0.7 MHz, A‘ = 1425.7 ± 2.3 MHz, (B‘ + C‘) = 590.6 ± 2.7 MHz. Furthermore, information about the alignment of the transition dipole moment in the molecular frame was obtained from the fit procedure. We report a geometry of the O−H···O hydrogen bonded phenol dimer as determined by a fit of the intermolecular parameters to the rotational constants. The ground-state results confirm the gross geometry of a former RCS investigation of Felker and co-workers [Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1992, 96, 2585]. Moreover, it was found that upon electronic excitation of the donor molecule the center of mass distance of the monomer moieties increases slightly from 5.25 Å ± 0.01 Å to 5.31 Å ± 0.05 Å. On the basis of assumptions for structural changes of the hydrogen bond and the donor monomer moiety upon electronic excitation, we propose a modification of the intermolecular structure of the phenol dimer, which is consistent with the experimental data. However, although the changes in the rotational constants are small, larger changes of intermolecular parameters cannot be excluded, e.g., a decrease of the wagging angle by several tens of degrees. The ground-state results are compared with structures obtained from calculations on different levels of theory. In particular, the results from semiempirical calculations based on atom−atom pair potentials and ab initio calculations at the MP2/6-31G(d) level of theory are examined.

Structural Characterization of 1:1 van der Waals Complexes of 9-Cyanoanthracene with Aprotic Solvents by Rotational Coherence Spectroscopy

Structures of 9-cyanoanthracene (CNA) clusters microsolvated with a single molecule of aprotic solvents (carbon dioxide, two isotopomers of acetonitrile, and fluoroform) have been studied by rotational coherence spectroscopy (RCS) implemented with the time-resolved fluorescence depletion method. All of the observed RCS traces exhibit pronounced C-type transients, and this fact suggests that these species are quite close to planar asymmetric tops with their electronic transition moments pointing to in-plane directions. Weak J-type transients have been also identified for CNA−CO2 and −CF3H, the latter of which shows A-type transients as well. By comparing the experimental observations with density functional theory calculations at the B3LYP/6-31G(d,p) level, it is concluded that the solvent molecule is located by the side of the CN group of CNA with its molecular axis lying in the CNA molecular plane. All of the cluster geometries are of Cs symmetry, in which a positively charged atom of the solvents (C, H, or H for CO2, CH3CN, and CF3H, respectively) is close to the cyano nitrogen of CNA, while an electonegative part (O, N, or F) contacts with the 1-position hydrogen of CNA. Some geometrical parameters including the centers of mass separation are obtained from the RCS-derived rotational constants.

Structural Characterization of 9-Cyanoanthracene−(Ar)n(n= 0−3) by Rotational Coherence Spectroscopy

Rotational coherence spectroscopy implemented with time-resolved fluorescence depletion has been applied in a structural study of 9-cyanoanthracene (CNA) and its clusters with Ar up to three atoms. For bare CNA, C-type transients for the S1 and S0 states have been observed separately, yielding independent sets of rotational constants for the two states. For the Ar clusters, rotational constants as averages for S1 and S0 have been derived to fix the cluster geometry. The Ar atom in CNA−Ar is located 3.46 ± 0.03 Å above the central aromatic ring of CNA and displaced slightly from the ring center toward the cyano group. The plane−Ar distance is quite close to those in clusters with other polycyclic aromatic molecules. Two values (∼0.2 or 0.6 Å) for the displacement to the cyano group are consistent with the experimental data, and results on related aromatics−Ar show that the former is preferable. The dominant conformer of CNA−(Ar)2 has been determined as a two-sided (1 + 1)-type: structures for each sides of the CNA plane are the same as that of CNA−Ar within the experimental uncertainties. CNA−(Ar)3 has a (2 + 1)-type structure: one side of the substrate is the same as CNA−Ar, and an Ar dimer lies 3.48 ± 0.04 Å above the other side. The determined conformations of CNA−(Ar)1,3 are the same as those of the corresponding anthracene clusters, but that of CNA−(Ar)2 is in contrast to that of anthracene−(Ar)2, which has been identified as a (2 + 0)-type. Model potential calculations have been employed to explain the difference in structural motifs of the two closely related clusters.

9,9′-Bianthryl and its van der Waals complexes studied by rotational coherence spectroscopy: Structure and excited state dynamics

The structure and excited state dynamics of jet-cooled 9,9′-bianthryl (BA) and its 1:1 van der Waals (vdW) complexes with Ne, Ar, and H2O were studied using rotational coherence spectroscopy (RCS). For a free BA molecule, the magnitude and persistence of the recurrent transient appearing in the time-correlated single photon counting (TCSPC) measurement was found to be dependent on the torsional level of BA, indicating the rotational constant changes with the torsional energy level. The RCS–TCSPC measurement of the BA–Ar and BA–H2O complexes in the S1 state showed no coherent transients. However, the pump–probe time-resolved fluorescence depletion (TRFD) detected the weak J-type transient. Those facts imply the loss of coherence in the BA vdW complexes due to the excited-state dynamics, which coincides with the analysis of the laser-induced fluorescence excitation and dispersed fluorescence spectra. The structure of the ground-state 1:1 BA complex with Ne, Ar, and H2O was determined based on the RCS transients observed in the TRFD measurement with the help of a minimum energy structure calculation using atom–atom pairwise potentials. The rapid dephasing in the excited state was demonstrated by the magic angle TRFD detection near t=0. The dominant dephasing process for the rare-gas complexes is ascribed to intramolecular vibrational energy redistribution (IVR) which is accelerated by significant coupling between the torsional vibration and the low-lying vdW vibrations. IVR process for the H2O complex accompanies the rapid conversion to the charge-transfer state, which is also responsible for the loss of excited-state coherence.

Rotational coherence spectroscopy of para-cyclohexylaniline by stimulated Raman-induced fluorescence depletion and stimulated emission pumping

A high-resolution rotational coherence spectroscopy (RCS) investigation of para-cyclohexylaniline (pCHA) was performed using the methods of time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD) and time-resolved stimulated emission pumping (TRSEP). TRSRFD and TRSEP are sensitive to ground state or excited state rotational constants, respectively and allow the deconvolution of the time-resolved fluorescence depletion (TRFD) spectrum to which both ground state and excited state rotational constants contribute. Moreover, from a detailed analysis of the presented experimental data it is deduced, that photoionization and internal vibrational relaxation (IVR)—not included in the picture of TRSEP, TRSRFD and TRFD—also contribute to the RCS spectra. The obtained rotational constants are in very good agreement with our previous high-resolution TRFD investigation of pCHA, allowing additionally for the unambiguous assignment of the excited state J-type transients. From a linear regression analysis of 25 transient positions (B+C)′=719.74(53) MHz was calculated, confirming set (II) of our former investigation [C. Riehn, A. Weichert, U. Lommatzsch, M. Zimmermann, and B. Brutschy, J. Chem. Phys. 112, 3650 (2000)].

High-resolution rotational coherence spectroscopy of para-cyclohexylaniline

A high-resolution rotational coherence spectroscopy (RCS) investigation of para-cyclohexylaniline (pCHA) was performed with a solid-state picosecond laser setup, which allowed for the determination of rotational constants with unprecedented precision for a RCS experiment. The technique of time-resolved fluorescence depletion was used for the RCS measurements. The unique structural features of pCHA enabled the determination of both ground and excited state rotational constants. Three different sets of recurrences were observed in the spectrum and assigned to K″-, K′-, and J″-type transients. From a detailed analysis by a grid search procedure based on the numerical simulation of RCS spectra and a nonlinear least-squares fitting routine the following rotational constants for the ground state were obtained: A″=2406.5±0.6 MHz, (B+C)″=714.9±0.4 MHz. For the electronic excited state two different sets of constants were found to fit the experimental data within the reported uncertainties: set (I) A′=2343.6±1.3 MHz, (B+C)′=714.4±1.7 MHz and set (II) A′=2346.3±1.3 MHz, (B+C)′=719.3±2.1 MHz. From additional information set (II) was found preferable for the description of the excited state. Furthermore, the fluorescence lifetime and the alignment of the transition dipole moment in the molecular frame were obtained from the fit procedure. For a structural characterization of pCHA we performed ab initio calculations of the electronic ground and excited state using HF/6-31G(d) and CIS/6-31G(d) levels of theory, respectively. These results were compared with the experiments and used to investigate the dependence of the rotational constants on characteristic intramolecular coordinates.

Rotational coherence spectroscopy of para-difluorobenzene–Ar

We report the rotational coherence spectra (RCS) of the aggregate para-difluorobenzene–argon using time-resolved fluorescence depletion (TRFD). A newly constructed, broad range tunable solid-state picosecond laser system has been employed. The rotational constants ( A+ B)=2.2346±0.002 GHz have been obtained in excellent agreement with former high-resolution UV spectra [R. Sussmann, R. Neuhauser, H.J. Neusser, Can. J. Phys. 72 (1994) 1179]. The spectra show contributions of ground as well as excited state rotational coherences and demonstrate the applicability of RCS to analyze the structures of medium-sized molecular systems (substituted benzenes) and clusters.

Rotational coherence spectroscopy of 9,9′-bianthyrl and its van der Waals complexes with Ar and H 2O

The excited state structure and dynamics of 9,9′-bianthryl and its Ar/H 2O complexes have been studied using rotational coherence spectroscopy (RCS). The intensity and the decay of the RCS signal were found to be dependent on the torsional level of the bianthryl, reflecting the change of rotational constants with torsional levels. For the Ar and H 2O complexes, no RCS signals were observed in a time-correlated single photon counting (TCSPC) experiment, though small signals were detected by a pump-probe time-resolved fluorescence depletion measurement. The smallness of the signal for the Ar complex is mostly attributed to structural reasons — deviation from a symmetric top and a hybrid-type transition unfavorable to RCS signals. For the H 2O complex, the loss of coherence due to the rapid conversion of the electronic state is responsible for the absence of the signal in the TCSPC measurements.

Study of the Ultrafast Processes of TPP,BuPc and NtBuPc

The ultrafast processes of tetra-phenyl-porphyrin(TPP), tetra-t-butyl-phthalocyanine(BuPc) and nitro-tri-t-butyl-phthalocyanine ( NrBuPc) have been investigated using the femtosecond time-resolved fluorescence depletion method. A sharp peak of the fluorescence depletion with delay time has been observed both for TPP and BuPc. An ultrafast two-photon absorption process has been proposed to explain the sharp peak observed. A long-lived fluorescence depletion has also been observed. It is causing by the stimulated-emission-pumping process produced by the intense probe laser pulses.

Theory of rotational coherence spectroscopy as implemented by picosecond fluorescence depletion schemes

We present a perturbation theory analysis of four time-resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground-state rotational constants determine the rotational coherence effects in fully resonant, time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD), excited-state rotational constants determine such effects in time-resolved stimulated emission spectroscopy (TRSES), and both ground- and excited-state constants do so in time-resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance-enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground-state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground- and excited-state rotational coherence effects are manifest in TRFD traces.

Temperature dependence of picosecond time resolved fluorescence depletion measurements

This paper presents the results of a temperature dependence study of time resolved fluorescence depletion (TRFD) measurements of intramolecular vibrational energy redistribution in the molecule p-cyclohexylaniline. TRFD scans of five vibrational bands of p-cyclohexylaniline were taken at several molecular beam conditions corresponding to rotational temperatures in the range of 8–110 K. The results are attributed to two possible coupling mechanisms and are shown to be consistent with previous work. Although rotational effects are probably dominant, our data also indicate that excitation of low (40 cm−1) vibrations may contribute to enhanced relaxation.

Picosecond fluorescence depletion spectroscopy. III. Intramolecular vibrational relaxation in the excited electronic state of p-cyclohexylaniline

This paper presents the results of the picosecond time-resolved study of intramolecular vibrational relaxation (IVR) in the molecule p-cyclohexylaniline. The results represent the second extensive study of a molecule using the technique which we have developed known as time-resolved fluorescence depletion (TRFD). Fluorescence depletion decays and dispersed fluorescence spectra of 19 S1 vibronic features are presented. The decays show a progression of dynamic behavior, including stationary behavior at low densities of states, quantum beating at intermediate densities, and fast decay of the initially prepared state at high state densities. The data allow us to assign IVR lifetimes which range from 250 ps at 820 cm−1 of excess vibrational energy to 2.2 ps at 2362 cm−1. Even at the highest frequencies we observe, the decays do not tend toward a simple exponential, but instead exhibit weak damped quantum beats.

Picosecond fluorescence depletion spectroscopy. I. Theory and apparatus

This paper describes the experimental implementation and theory of a new technique for the time resolved study of intramolecular vibrational redistribution: time resolved fluorescence depletion (TRFD). Picosecond time resolution is obtained by using two pulses of identical wavelength and duration to create and then stimulate emission from a vibrational state in S1. Spontaneous emission intensity is measured as a function of delay between pulses. By monitoring the efficiency with which the second pulse stimulates emission, the evolution of the initial optically prepared state can be followed. A description of the apparatus built for these experiments is also presented.