With this award, co-funded by the Division of Chemistry and the Major Research Instrumentation (MRI) program, Professor Igor Rubtsov from Tulane University will develop a robust, user-friendly, fully-automated two-dimensional (2-D) infrared spectrometer that can be used by non-specialists. The PI has a number of collaborators who are interested in using 2D-IR spectroscopy to study the vibrational spectra of medium to large molecules. The instrument will use short infrared pulses to access various vibrational transitions in a given molecule to measure couplings of the vibrational modes, angles between the transition dipoles, and correlate vibrational frequency distributions.
The academic research enterprise relies on new generations of sophisticated research instruments. The instrument will generate vibrational data which are important to understand physical and spectroscopic properties of molecules. In addition, the instrument will serve as prototype for others that will be useful for universities, government laboratories and companies. Student participation will promote formation of the new generation of instrument builders.
Two-dimensional infrared (2DIR) spectroscopy emerged about a decade ago as a new tool for measuring three-dimensional structures of molecules in solution and in solid phase. The method measures pair-wise couplings (interaction strengths) of vibrational modes in molecules – the coupling strength reports on the distance between the vibrating groups thus producing a bit of structural information. The coupling is determined from the strength of the cross-peak in the 2DIR spectrum; by combining many bits of structural data the 3D structure of the molecule can be determined. Many vibrational modes in molecules are localized on particular groups, which make them perfect reporters for the molecular structure. 2DIR spectroscopy has demonstrated its power for measuring structures of numerous compounds including drugs, peptides, transition metal complexes, and recently proteins. Wider implementation of this powerful method is constricted by absence of a commercial instrument capable of accessing the entire 2DIR spectrum. Recent developments of Zanni group at the University of Wisconsin-Madison made available a so called single-color 2DIR spectrometer. Note, however, that mid-IR laser pulses have a typical width of 200 cm-1, so the single-color approach permits measuring structural constraints based only on modes that are close in frequency, which is a serious limitation. Dual-frequency 2DIR uses two independently tunable mid-IR pulses and has no limitations on the mode frequencies. Moreover, often unwanted diagonal peaks can be strongly reduced or fully eliminated using dual-frequency 2DIR. The objective of this NSF project was to design and build a user-friendly dual-frequency 2DIR spectrometer, which can be used by non-specialists in ultrafast spectroscopy. This goal has been accomplished. We have designed and built the first fully automated dual-frequency 2DIR instrument that permits measuring any cross peak within a spectral range from 800 to 4000 cm-1. The instrument is designed to measure weak cross peaks and its sensitivity, therefore, is of great importance. We have implemented the most sensitive approach to measure cross peaks: a three-pulse photon-echo method with heterodyned detection. As a result, the instrument has superior sensitivity that reaches 10-3 cm-1 in measured couplings, which is achieved by a combination of spectral interferometry, phase cycling, and closed-loop active phase stabilization (Image 3) accurate to 70 attoseconds (70x10-18 s). The automatic frequency tuning is achieved by implementing beam direction stabilization schemes for each mid-IR beam, providing beam stability better than 50 mrad (Image 4), and novel scheme for setting the phase-matching geometry for the mid-IR beams at the sample. The instrument is fully computer controlled and, therefore, can be used by non-specialists in ultrafast spectroscopy. Images 1 and 2 show examples of 2DIR spectra acquired with the new instrument. The spectra are composed of several spectral regions, each covering about 250x250 cm-1 and taking 3-5 min to acquire. Acquisition of the entire 2DIR spectrum was performed in a fully automatic regime. Transition from one spectral region to another requires about 5 min. This time is needed to change the central frequencies of the two mid-IR beams, stabilize their directions, change the geometry of the beams interacting with the sample, change the oscillation amplitude for the phase-cycling devices, change the monochromator wavelength, change the delay between mid-IR pulses of different frequencies, adjust the intensity of the local oscillator, and recover a p/2 phase shift between two detectors for active phase stabilization (Image 3). Most of the changes are made automatically, but some, such as setting the monochromator wavelength and intensity of the local oscillator, require input of the operator at the computer. The most time consuming step is associated with stabilization of the beam direction, which takes about 1.5 minutes for each beam. The overall performance of the instrument was found to be excellent: automated measurements of the 2DIR spectra allowed achieving the alignment quality within 10% of the optimal alignment obtained manually. The detailed description of the performance is reported in two manuscripts: one covering the beam direction stabilization schematic was submitted to Optics Express journal and another describing the whole instrument is currently at the final stage of preparation. The results were reported at several conferences. Five graduate and two undergraduate students were trained during the project – four of them reached the designer level in 2DIR. The dual-frequency 2DIR spectroscopy method, available only in three research laboratories in the world, is now accessible for use by researchers not specializing in 2DIR spectroscopy. This accessibility will allow for the analytical power of the method to be revealed more quickly, permitting it to make contributions to various fields of chemistry. One of the first research projects started on the new instrument is in collaboration with Prof. Jonathan Sessler (UT Austin), who visited the PI’s lab in November 2013. The website announcing the instrument availability and the principles of using it is set up within the PI lab homepage, https://tulane.edu/sse/chem/faculty/igor-rubtsov/index.cfm.
