Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems

Our protocol is particularly powerful for looking into the structure and dynamics of pi-conjugated molecules in the initial stage of chemical and biochemical reactions. This technique has a little bit higher sensitivity and makes the measurement time shorter than conventional time-resolved spontaneous Raman spectroscopy in the near-infrared with detectors available today. To begin, complete the optical setup as shown here.

First, align the laser beam. With the titanium sapphire laser turned on and warmed up, place a business card behind iris two to act as a screen. Adjust mirror one until the beam passes through the center of the iris.

Then, adjust mirror two until the laser beam passes through the center of iris two. Once aligned, confirm that the beam passes through the centers of both iris one and iris two simultaneously. With the laser properly aligned, begin to align the optical delay line.

First, move the stage toward mirror two as far as it can go using the direction button of the stage controller. Next, adjust mirror one until the beam passes through the center of iris one. Then, move the stage as far away as it can go from mirror two.

Adjust mirror two until the beam passes through the center of iris one. Now, move the stage as close as possible to the beam input and confirm that the beam still passes through the center of iris one. Next, remove iris one from the position of mirror three and place mirrors three and four on the optical delay line.

Adjust mirrors three and four until the beam passes through the center of iris two. With the variable neutral density filter in the incident beam path, place a business card behind the sapphire plate as a screen. Turn the filter to gradually increase the power of the transmitted beam until a yellow white spot is observed on the screen.

Then, turn the filter further in the same direction very carefully until a purple ring surrounds the yellow white spot on the screen. Next, to align the Raman pump beam, place the volume grading reflective bandpass filter in the output beam path of the optical parametric amplifier. Adjust the bandpass filter and mirror 17 using a near-IR sensor card in order to observe the beam spot.

To begin optimizing the probe spectrum, run continuous measurements and maximize detector counts on the display. To accomplish this, gradually rotate half wave plate one. Then, gradually increase the intensity of the incident pulse by rotating variable optical density filter one.

Do this until the maximal and minimal detector counts reach around 30, 000 and 4, 000 respectively. If a large oscillatory pattern starts to be observed, rotate the variable optical density filter in the opposite direction until the pattern disappears. For set up for spatial overlap, place the optical chopper in the Raman pump beam path.

Then, place a near-IR sensor card at the sample position. Adjust the direction of the Raman pump beam by adjusting mirror 21 until the spots of the Raman pump and probe beams fully overlap with each other. To set up for the temporal overlap, place an indium gallium arsenide pin photodiode at the sample position where the Raman pump and probe beams spatially overlap with each other.

Next, connect the signal output of the photodiode to a 500 megahertz five giga samples per second digital oscilloscope in order to monitor when the Raman pump and probe pulses arrive at the same position. Set the horizontal scale of the oscilloscope to be one nanosecond per division and read the peak time of the signal intensity for the Raman pump and probe pulses blocking the other pulse. Attach the inlet and outlet tubes of the magnetic gear pump to a bottle containing 30 milliliters of cyclohexane and begin flowing cyclohexane as described in the text protocol.

Run continuous measurements and check if the stimulated Raman bands of cyclohexane are observed in the display. The strongest band of cyclohexane appears at the 55th through 58th pixels when the center wavelength is set at 1, 410 nanometers. Once the stimulated Raman bands are detected, maximize the band intensities in the display.

Accomplish this by iteratively readjusting mirror 21, the rotational phase of the optical chopper, and the position of optical delay line two. Run a single measurement and save the spectrum as a text file. Then, remove the toluene from the reservoir and attach the inlet/outlet tubes of the magnetic gear pump to a bottle containing 25 milliliters of a toluene solution with 1X times 10 to the negative four moles per liter of beta-carotene.

Then, start flowing the sample solution. Next, place the optical chopper in the actinic pump beam path. Move the beam dump from the path of the actinic pump beam to that of the Raman pump beam.

Then, spatially overlap the actinic pump and probe beams at the sample position using a business card instead of the near-IR sensor card. Run continuous measurements and check if the transient absorption of beta-carotene is observed in the display. The absorption band appears with a shape decreasing monotonically toward longer wavelengths or with two maxima at around the zero and 511th pixels.

Maximize the absorption intensity by readjusting mirror 32 once the transient absorption band is detected. Stop the continuous measurements and then decrease the position of the optical delay line one until the transient absorption fully disappears. Place the optical chopper in the Raman pump beam path and remove the beam dump from the Raman pump beam path.

Then, run a time-resolved experiment as described in the text protocol selecting SK stage from the dropdown menu. Enter the starting position of range A to be smaller by around 50 microns compared to the position where the transient absorption signal disappeared following the measurement of time-resolved absorption spectra. Femtosecond time-resolved near-IR stimulated Raman spectroscopy was applied to beta-carotene and toluene solution.

The spectra of beta-carotene and toluene is shown here. The raw spectra contained strong Raman bands of the solvent toluene and a weak Raman band of beta-carotene in the ground state as well as Raman bands of photoexcited beta-carotene. Shown here are the same spectra but subtracted using the stimulated Raman spectrum of the same solution at one picosecond before photoexcitation.

The spectra after the subtraction showed distorted baselines that are caused by absorption of photoexcited beta-carotene and/or other nonlinear optical processes. The baselines became flat after they were corrected with polynomial functions. In this figure, the time-resolved stimulated Raman spectra of beta-carotene showed two strong bands in the 1, 400 to 1, 800 inverse centimeter region.

A broad stimulated Raman band at zero picoseconds was assigned to the in-phase C double bond C stretch vibration of S2 beta-carotene. Its peak position was estimated to be at 1, 556 inverse centimeters. The in-phase C double bond C stretch band of S1 beta-carotene appeared as the S2 C double bond C stretch band decayed.

The peak position of the S1 C double bond C stretch band was upshifted by eight inverse centimeters from 0.12 to five picoseconds. It is important to try and adjust the direction of the Raman pump beam again and again until the spots of the Raman pump and probe beams fully overlap with each other to find the stimulated Raman bands of cyclohexane. This procedure can be immediately used for other femtosecond time-resolved experiments in order to look more deeply into chemical reaction dynamics.

This procedure will allow for new questions to be answered as researchers explore the chemistry of pi-conjugated molecules. While performing this procedure, don't forget to put safety glasses on to protect your eyes from strong laser light. This includes the scatter light.