Andreas Weichert

Rotational coherence spectroscopy of benzene by femtosecond degenerate four-wave mixing

Femtosecond time-resolved non-resonant degenerate four-wave mixing (fs-DFWM) experiments were applied to study the rotational coherences in benzene molecules in the gas phase at room temperature. The experiments were performed in a cell filled with benzene vapour at reduced pressure (0.01 bar). The technique gives an excellent signal-to-noise ratio for the observed transients. For the first time transients up to a maximum time delay of 1.4 ns, mainly limited by collisional dephasing, have been obtained for a molecule of this size by fs-DFWM. From this data the rotational constant B = 0.18972 ± 0.00010 cm−1 for benzene at room temperature is obtained by a fitted simulation. The application of fs-DFWM to large molecules seems to be possible and might produce some interesting results in the future.

Rotational coherence spectroscopy of para-difluorobenzene–Ar

Abstract 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.

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 M...

Implementation of a high-resolution two-color spectrometer for rotational coherence spectroscopy in the picosecond time domain

A high-resolution two-color spectrometer for rotational coherence spectroscopy (RCS) in the picosecond time domain has been developed and applied to several molecular systems. This time-resolved method enables the structural characterization of large molecules and clusters in the gas phase by determination of their rotational constants with high precision. Our spectrometer is based on a well stabilized solid-state chirped pulse amplified laser setup with an adjustable pulse duration in the picosecond range. Two optical parametric generators provide tunable radiation in the UV spectral range, with a pulse duration of 2 ps. An interferometric setup is employed in order to perform pump–probe experiments with up to 8 ns delay between pump and probe pulse. Two different pulse to pulse switching modes and the online monitoring of laser parameters have been integrated to enable the measurement of RCS traces with high resolution and good signal/noise ratio. These modes also permit the identification of weak featu...

Mass-selective rotational coherence spectroscopy by (1 + 1′) pump–probe photoionization

We report the first experimental results on a simple scheme for mass-selective rotational coherence spectroscopy (RCS) employing a (1 + 1′) UV pump–probe photoionization setup. A solid state two-color UV ps laser system, optimized for high-resolution RCS, provides two independently wavelength tunable, linearly polarized output pulses. The pump pulse resonantly excites a sample and creates a superposition of rovibronic states at time zero. After a variable delay time a probe pulse, non-resonant to transitions in the neutral manifold of the sample, is applied for photoionization and the ion-yield is recorded. By this method sharp and intense J-type transients for the molecule para-cyclohexylaniline (pCHA) have been identified. Correspondingly, transients of opposite polarity have been obtained by detection of fluorescence from the first electronic excited state. The transient positions allow the determination of the excited state rotational constants of pCHA. The obtained values confirm the results of our former high-resolution fluorescence depletion RCS study of pCHA. Moreover, due to its relative simple implementation and mass-selectivity, the presented photoionization setup bears great potential for future applications in the structural analysis of molecular clusters.

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 o...

Time-resolved rotational spectroscopy of para-difluorobenzene·Ar

Abstract We report on time-resolved rotational spectroscopy experiments of the cluster para -difluorobenzene·Ar ( p DFB·Ar) by picosecond laser pulses in a supersonic expansion. Rotational coherences of p DFB·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 p DFB·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 p DFB monomer, is presented resulting in a p DFB·Ar center-of-mass distance of both moieties of R z =3.543±0.017 A with a change of Δ R z =−0.057±0.009 A upon electronic excitation. These results are compared to data of former frequency-resolved experiments and ab initio computations.

Chapter 15 - Different schemes and recent results for high-resolution rotational coherence spectroscopy with picosecond and femtosecond laser pulses

For pump-probe photoionization (PPI) the first laser pulse is tuned into resonance with the (vibration-less) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (l+1’’) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection, the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. Time-resolved degenerate four-wave mixing (TRDFWM or femtosecond DFWM), can be envisaged as a transient grating spectroscopy. The first two laser pulses generate a grating of coherently Raman excited molecules in the electronic ground state. The time dependence of this grating of aligned molecules is probed by diffraction of a third delayed laser pulse. The process is non-resonant, so that neither chromophore nor dipole moment has to be present in the molecule under study. It provides background free detection so that a high signal/noise ratio (up to 10 4 ) can be achieved.

State-selective rotational coherence spectroscopy by two-color pump-probe schemes

Rotational constants of para-cyclohexylaniline in the electronic ground and excited state were measured in the gas phase by time-resolved Raman spectroscopy and stimulated emission pumping. A new mass-selective scheme by pump-probe photoionization is presented.