Experimental Methods

The 2DRR experiments conducted in this work utilize either five or three laser beams to obtain the fifth-order response. Measurements conducted in these geometries must contend with a background of residual laser light and/or lower-order nonlinearities, because fewer than six laser beams are employed.26 In this section, we describe the two experimental setups and discuss how sources of background are dealt with.

IIIA. Conducting 2DRR Spectroscopy with a Five-Beam Geometry

Detection of signal components described by terms 5-8 in Figure 2.2 is accomplished with a geometry of five laser beams. In Figure 2.4, it is shown that a 340-nm laser beam is simply added to an existing diffractive optic-based transient grating setup operational at 680 nm.40 A slightly modified version of this interferometer has been described elsewhere.41,42 Briefly, the 680-nm beams are focused on the diffractive optic with a 20-cm focal length

Thus, a square pattern of 680-nm beams appears on the 20-cm focal length spherical mirror. The spherical mirror is tilted off-axis by approximately 5º (i.e., the minimum amount) in order to image the spot from the diffractive optic onto the sample. Focusing conditions of the 340-nm beam are optimized to match the 200-μm FWHM spot sizes of the 680-nm beams.

The 340-nm and 680-nm laser beams are produced by focusing a 0.8-mJ, 60-fs laser beam at 800 nm into a 43-cm long hollow core fiber with a 250-μm inner diameter. The

continuum produced in the fiber spans the full visible spectral range. A 4-uJ, 40-nm wide portion of the continuum centered at 680 nm is filtered in a fused silica prism compressor. Most of the 680-nm beam (65%) is used to generate 340-nm light in a 100-micron thick, Type I Beta Barium Borate (BBO) crystal. In order to minimize lossy reflections, the 340-nm beam is directly imaged from the BBO onto the sample using a 15-cm focal length spherical mirror placed 30-cm from the BBO. Residual 680-nm light is filtered using a 1-mm thick fused silica polarizer. A lossy second compression step is not required for the 340-nm beam because of precompensation for dispersion in the aforementioned prism compressor; the polarizer used to filter residual 680-nm light compensates for negative chirp in the 340-nm pulse.

Figure 2.4. (a) Diffractive optic-based interferometer used to detect signal components

described by terms 5-8 in Figure 2.2. Each of the two 680-nm beams is split into -1 and +1 diffraction orders with equal intensities at the diffractive optic. The signal is collinear with the reference field (pulse 5) used for interferometric signal detection. (b) The 340-nm pulse induces photodissociation and vibrational coherence in the diiodide photoproduct during the delay, ₁. The time-coincident 680-nm pulses, 2 and 3, reinitiate the vibrational coherence in diiodide during the delay, ₂.

In this pulse sequence, the 40-fs, 340-nm pulse (pulse 1 in Figure 2.4) induces a photodissociation process that leaves the diiodide photoproduct in a vibrational coherence as suggested by terms 5-8 in Figure 2.2. A time-coincident pair of 25-fs, 680-nm laser pulses (pulses 2 and 3 in Figure 2.4) reinitiates the vibrational coherence in diiodide during the delay,

2

 . The fourth pulse (also at 680 nm) induces signal emission. The fifth pulse, which is attenuated by a factor of 1000 before the sample, is used for heterodyne detection by spectral interferometry.43,44 The signal phase can be determined using the method devised by Turner and

Scholes in this beam geometry,45 _{because the 340-nm pulse does not factor into the phase} calibration. Scherer and Blank have employed similar laser beam geometries and phasing schemes in related fifth-order experiments.46-48

An undesired four-wave mixing response may be radiated by the solvent in the same direction as the fifth-order signal in this geometry. However, because the sample is transparent at 680 nm, this four-wave mixing signal is approximately 50 times smaller than that associated with the solute at delay times greater than 80 fs. Moreover, the desired signal radiated by the solute exhibits a vibrational coherence with a period of 300 fs. Insensitivity of this setup to

intramolecular vibrations of the solvent was confirmed by scanning the delay, ₂, with the 340 nm beam blocked. Thus, the assignment of the experimentally observed 112-cm-1 _vibrational resonance to the solute is unambiguous.26 The 2DRR experiment may be conducted without chopping the 340-nm beam, because the desired fifth-order nonlinearity dominates the total response of the solution. Conducting the experiment without chopping the 340-nm beam greatly speeds up data acquisition and facilitates signal averaging.

Signals are detected using a back-illuminated CCD array (Princeton Instruments PIXIS 100B) mounted on a 0.3 meter spectrograph with a 600 g/mm grating. The signal generates roughly 80 counts/ms on the detector with 150-nJ, 340-nm pulses and 200-nJ, 680-nm pulses. All beams possess the same electric field polarization and are focused to 200 μm at the sample position. The two delay lines are scanned 20 times and averaged. The step sizes are 40 fs in both dimensions.

IIIB. Conducting 2DRR Spectroscopy with a Three-Beam Geometry

Signal components of the type described by terms 9-12 are detected using a three-pulse geometry (i.e., a standard pump-repump-probe experiment).49 As shown in Figure 2.5, the first

two pulses that arrive at the sample are 25-fs, 400-nm pulses produced by self-phase modulation in a hollow core fiber,50 whereas the third pulse is a visible continuum produced in a 3-mm thick sapphire plate. The 400-nm beams are focused onto the sample with a 30-cm focal length

spherical mirror, whereas the continuum is relayed from the sapphire plate onto the sample using a single 5-cm focal length mirror (the continuum focuses 35 cm from the spherical mirror). The FWHM spot sizes of the 400-nm beams are 600 μm, whereas that of the continuum is 400 μm. Angles between the adjacent beams are 5°. Pulse energies of the 400-nm beams range from 150- 300 nJ in various experiments, and we observe no differences in the vibrational lineshapes obtained within this range of pulse energies. The phases of the two chopper wheels, which are both operated at 250 Hz, are shifted by 90° to acquire signals under the four conditions needed to produce a pump-repump-probe signal (A).49 Signal detection is accomplished with a CMOS array detector that is synchronized to the 1-kHz repetition rate of the laser system. The noise level of a pump-repump-probe signal is approximately 0.1 mOD in this setup. The delay lines are scanned 10 times with step sizes of 40 fs and averaged.

Figure 2.5. (a) Pump-repump-probe beam geometry used to detect signal components described

by terms 9-12 in Figure 2.2. (b) The first 400-nm pulse promotes a stimulated Raman response in the ground electronic state of the triiodide reactant during the delay, ₁. The second pulse

induces photodissociation of the non-equilibrium reactant, thereby giving rise to vibrational coherence in the diiodide photoproduct during the delay, ₂. Sensitivity to diiodide is enhanced by signal detection in the visible spectral range.

Two field-matter interactions with triiodide occur with each of the 400-nm pump pulses in this experiment. The first pulse stimulates wavepacket motion in the ground electronic state of triiodide as indicated in terms 9-12. The application of a second 400-nm pulse ensures that the signals are primarily sensitive to vibrational coherences of triiodide during 1 (i.e., signal contributions from diiodide are negligible during ₁). The key issue is that the transient

electronic resonance of triiodide is dominant at 400 nm (i.e., the bleach of the ground state). The second 400-nm pulse induces photodissociation of triiodide and leaves the diiodide photoproduct in a vibrational coherence in ₂. Signal detection in the visible spectral range enhances

IIIC. Sample Preparation and Handling

Triiodide solutions are prepared by mixing solid I2 (Aldrich) with 5-fold molar excess of KI (Aldrich) in ethanol (Decon Labs, 200 proof). The solutions are stirred for one hour to fully dissolve the solid. The absorbance of the solutions is equal to 0.5 at 400 nm in a 300-μm path length. The sample is flowed through a wire-guided jet with a thickness of 300 μm, where the volume of the reservoir is 50 mL.