A relaxation-assisted 2D IR spectroscopy method

Kurochkin et al. 10.1073/pnas.0700560104.

Supporting Information

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SI Figure 10
SI Experimental Methods




SI Figure 10

Fig. 10. Magnitude of dual-frequency slices of the (wt, T) spectrum for cyanocoumarin, measured with a 4-, 4-, 6-mm pulse sequence taken at T-delays of 0.67, 3.0, 5.3, 10.7, and 13.3 ps.





SI Experimental Methods

Dual-Frequency Heterodyne 2D IR. Short pulses in the mid-IR spectral region required for 2D IR experiments were produced by using laser pulses at 804 nm and 44 fs in duration, generated by a Ti:sapphire oscillator and a regenerative amplifier system (Coherent, Santa Clara, CA). The output of the regenerative amplifier (900 mJ per pulse) was split into two equal parts to pump two in-house-built optical parametric amplifiers, each equipped with a difference frequency generation (DFG) unit. Two independently tunable mid-IR pulse trains were generated by the two DFG modules that use 1.5-mm-thick AgGaS2 crystals (q= 37.1°; Ekspla, Vilnius, Lithuania). The resulting pulses had the energy of 1.8 and 2.5 mJ per pulse in the 6- and 4-mm spectral regions, respectively; a duration of ≈90 fs; and bandwidth of 176 ± 5 cm-1 throughout the spectral region.

Each mid-IR beam was split into two equal parts to provide k1, k2, k3, and local oscillator (LO) beams. The k1, k2, k3 pulses were focused onto the sample with a 51-mm focal length, gold-coated, off-axis parabolic reflector with the pulse geometry shown in Fig. 9. The phase-matching conditions for the dual-frequency experiments lead to a trapezoid spot pattern in the plane crossing the beams after the sample cell, so that the beams with higher frequency hit the trapeze shorter-base corners (Fig. 9). The third-order signal generated in the sample was picked at the phase-matching direction (-k1 + k2 + k3) by another parabolic reflector and mixed with the LO on an MCT detector (Infrared Associates, Stuart, FL). The time-delays t, T, and t between the pulses (Fig. 9) were controlled by linear motor-driven translation stages (Physik Instrumente, Karlsruhe, Germany). The pulse spectra were measured, and the relative intensities at various wavelengths were corrected for the monochromator dispersion.

In most experiments, the pulse spectra were tuned so that the k1 and k2 beams were at »4 mm, matching the spectrum of the CN peak, and the k3 and LO beams were at 6 mm, overlapping with the CO peak (Fig. 2). The 2D (t, t) spectra were recorded by using the 8.7- and 7.0-fs steps in the t and t directions, respectively, and were double Fourier-transformed to obtain (wt, wt) spectra. Experiments of two types were measured: rephasing and nonrephasing. Rephasing experiments were collected when the k1 beam arrived first and the k2 beam arrived second; for nonrephasing spectra, the order of the two beams was reversed. The polarizations of all four beams were kept vertical in this study, controlled by pairs consisting of a half-wave plate (Karl Lambrecht, Chicago, IL) and a BaF2 wire-grid polarizer (Specac, Woodstock, GA). Absorptive 2D spectra were constructed as a sum of real rephasing and nonrephasing spectra.

Dual-Frequency Heterodyned (wt, T) Spectra. Series of t-scan, dual-frequency interferograms were measured as a function of T waiting time, while the delay time t was slowly varying from 20 to 660 fs, ensuring rephasing conditions. The spectra were then Fourier-transformed along the t direction and presented as a set of 1D wt spectra at various T values. Note that a peak in the wt spectrum includes contributions from all of the peaks seen at the same wt frequency in the 2D (wt, wt) spectrum measured under the same experimental conditions. This type of measurement allows fast detection of the cross-peak amplitude as a function of T-delay time. Small monotonic variations in t were brought by a 1.2% difference in the unit step for the two motors used to set the delay of the k1 and k2 pulses. Introduced accidentally, the variation of the t value appeared to be very advantageous because it allowed the separation of different contributions along the wt axis in simple 1D wt scans.

Pump-Probe IR Measurements. IR pump-probe measurements at 4 mm were performed to study excited-state population dynamics of the CN mode in 3-cyanocoumarin. The probe beam passed through the sample was dispersed by a monochromator (TRIAX-190; HORIBA Jobin Yvon, Edison, NJ) and recorded by the MCT detector.

This Article

  1. Published online before print June 8, 2007, doi: 10.1073/pnas.0700560104 PNAS September 4, 2007 vol. 104 no. 36 14209-14214
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