STRUCTURE OF LARGE MOLECULES AND CLUSTERS BY TIME-RESOLVED ROTATIONS

The rotational periods of freely rotating molecules and molecular aggregates in the gas phase are measured in “real-time” by fluorescence or photoionization detection.

Fig. 1 Principle of pump probe rotational coherence spectroscopy (RCS).

From the characteristic rotational times the moments-of-inertia of the species under study are determined and the molecular structure is deduced. A close colloboration between experiment and theory is essential for the interpretation (simulation and fitting) of the RCS spectra and important for determination and comparison of the molecular structure.

Pump-probe RCS(Fig. 1) can be envisaged as follows: at time zero a linearly polarized pump pulse excites molecules that are aligned with their transition dipole moment parallel to the laser polarization. After a short time (~average rotational speed), this initial alignment is lost since the different molecules rotate at different rotational speeds. However, after characteristic time periods, which are related to the rotational constants, a reccurence of the original alignment is taking place. This recurrence is probed by a second time delayed laser pulse. Pioneering experiments for RCS have been performed by Baskin, Felker and Zewail (1986).[1,2]

As an example of our work the RCS spectrum of p-cyclohexylaniline, obtained by time-resolved fluorescence depletion, is given in Fig. 2.[3]

Fig. 2 RCS of para-cyclohexylanline by time-resolved fluorescence depletion (TRFD). Black: experimental spectrum; red: fitted simulation.

From the fitted simulation of the spectra we can obtain the rotational constants of, both, the ground (S₀) and electronically excited state (S₁) with high-resolution, the alignment of the transition dipole moment and the lifetime of the excited state.[4]

Tab. 1 Rotational constants [MHz] for p-cyclohexylaniline S0 / S1.

Fig. 3: Structure of para-cyclohexylaniline (S0).

In order to determine the “speed” of chemical dynamics on a molecular scale, that is the nuclear rearrangements, the making and breaking of bonds and the vibrations and rotations of molecules, they are naturally followed in the time domain. Of course, complementary information can be inferred from the width of rotationally resolved spectroscopy in the frequency domain.

One of our goals is to obtain structural information (rotational constants) on large molecules and molecular clusters (>20 atoms). Here, the rotationally-resolved frequency domain techniques (microwave, rot. res. IR, rot. res. UV) suffer from the strongly increasing density of spectral lines resulting from the decrease of the related rotational constants. In the time domain the rotational periods are increasing with increasing molecular size and the rotational constants can be obtained precisely for large systems (relative resolution 10-5). An example is given by the phenol dimer (Fig. 4), which was investigated by RCS but could not be analyzed by frequency domain techniques.

The timescales of molecular chemical dynamics are in the range of femtoseconds (10^-15 s) for vibrations to picoseconds (10^-12 s) for rotations. In order to explore a molecular chemical event in “real-time”, a device with an ultrafast time-resolution is required. Electronic devices cannot provide this resolution, so that an all-optical setup with ultrashort laser pulses is used.

The common laser technique applied to study ultrafast processes is called: pump-probe spectroscopy (Fig.5).

Fig. 5 Setup for ultrafast pump-probe spectroscopy.

The first ultrashort laser pulse pumps an electronic or vibrational transition in a molecule and the subsequent response of the system will be detected by a following ultrashort probe laser pulse. The probe pulse can be continously delayed in time with respect to the pump pulse with a translation stage (in 1 ps light travels 0.3 mm, in 1 s the distance between the earth and the moon). By ultrafast pump-probe spectroscopy one can observe the “birth of a new molecule” as it was coined by A. H. Zewail, Nobel prize winner in 1999 for “Femtochemistry”.

As a probe in the gas phase, laser-induced fluorescence or photoionization coupled to time-of-flight mass spectrometry are used. The wavelengths of pump and probe laser pulses have to be carefully adjusted in order to excite and detect the species of interest. Our ultrashort laser system (Fig.6) can be configured to deliver picosecond (2ps, 250-400 nm, 30 µJ) or femtosecond (150 fs, 800 nm, 1 mJ) laser pulses with a repetition rate of 1 kHz.

In our applications we make use of the coherence, polarization and time-bandwidth product of ultrashort laser pulses.

The coherence in time allows us to define time zero of a chemical event and excite a manifold of molecules within an ultrashort time span corresponding to the duration of the pulse.

Linearly polarized laser light enables us to select and detect molecules in a defined spatial orientation. The molecules can be envisaged as small antennas that can be preferentially excited when oriented parallel to the polarization of the laser pulse.

The time-bandwidth relation (Fourier limit) states that the product of the laser pulse duration and the spectral bandwidth are constant and therefore ultrahort laser pulses are accompanied by a broad spectral width (1 ps pulse duration equals a spectral width of ca. 15 cm^-1 in the UV). This spectral broadness is used to generate a superposition state composed of different energy levels of the molecules. Interestingly, this quantum superposition state (vibrational or rotational wavepacket) behaves closely like a classical object (oscillator or rotor) and can therefore be described in a straightforward way.

A transient grating of rotationally Raman-excited molecules is generated in the overlap region of two pump laser pulses in the gas phase. A third delayed probe laser pulse is diffracted of this grating and a coherent signal is detected under a predetermined angle at characteristic rotational times, similar to the RCS technique.

Fig. 6 Fs DFWM spectrum of formic acid (10 mbar, 300 K)

Features of fs DFWM are: excellent signal/noise; determination of ground state rotational constants; precise data due to fs time resolution; supersonic jet (30 K) and gas cell (300 K) experiments possible; non-resonant scheme, no dipole moment or chromophore necessary. [4,5]

Fig. 7 Fs DFWM spectra of the formic acid dimer (HCOOH)2.

As an example of our work the fs DFWM spectra of formic acid monomer and dimer are given in Figs. 7 and 8. From the fitted simulation we obtained the rotational constants, the centrifugal distortion constants and information on the polarizability tensor.

Pump-probe photoionization allows for the mass-selective (cluster specific) and time-resolved investigation of processes in the electronically excited state. By this technique RCS, vibrational coherence or chemical dynamics of charge or hydrogen transfer in the electronically excited state can be probed.

More information can be gained by time-resolved photoelectron spectroscopy which allows for the direct mapping of the change of electronic structure during a chemical process in the excited state. We have set up a 3D electron imaging spectrometer with 4? detection. Energetic and angular distributions of the photoelectrons can be determined.

As an example, a photoelectron imaging spectrum of p-difluorobenzene is depicted in Figs. 8 a,b.

pDFB+ levels      photon signal

Fig. 8 3D photoelectron imaging of para-difluorobenzene. Laser polarization perpendicular to detector plane.
a) Y coordinate of detector versus electron time-of-flight.
b) Y coordinate of detector versus X coordinate of detector.

[1] J. S. Baskin, P. M. Felker, A. H. Zewail,
Doppler-free time-resolved polarization spectroscopy of large molecules: Measurement of excited state rotational constants,
J. Chem. Phys. 84 (1986), 4708-4710.

[2] P. M. Felker,
Rotational Coherence Spectroscopy: Studies of the Geometries of Large Gas-Phase Species by Picosecond Time-Domain Methods,
J. Phys. Chem. 96 (1992) 7844-7857.

[3] C. Riehn, A. Weichert, U. Lommatzsch, M. Zimmermann, B. Brutschy,
High-Resolution Rotational Coherence Spectroscopy of para-Cyclohexylaniline,
J. Chem. Phys. 112 (2000) 3650-3661.

[4] A. Weichert, C. Riehn, B. Brutschy,
Rotational coherence spectroscopy of para-cyclohexylaniline by stimulated Raman-induced fluorescence depletion and stimulated emission pumping,
J. Chem. Phys. 113 (2000) 7830-7837.

[5] C. Riehn,
High-resolution Pump-Probe Rotational Coherence Spectroscopy - Rotational Constants and Structure of Ground and Electronically Excited States of Large Molecular Systems,
Chem. Phys. 283 (2002) 297-329.

[6] V. V. Matylitsky, W. Jarzeba, C. Riehn, B. Brutschy,
Femtosecond Degenerate Four-Wave Mixing Study of Benzene in the Gas Phase,
J. Raman. Spectrosc. 33 (2002) 877-883.

Vacuum apparatus for picosecond rotational coherence, time-resolved photoionization and photoelectron spectroscopy.

Ultrashort laser setup for two-color pump probe experiments.

Picosecond TOPAS unit. Optical parametric generator/amplifier pumped by second harmonic of CPA system (400 nm). Computer-controlled tunable: 500-700 nm (SHG 250-350 nm).