Figure 9.1: Instrument schematic of the proposed MW-THz double resonance instrument. Rotational states of molecular clusters are pumped with a continuous wave microwave source and monitored with the THz frequency comb.
reflection geometry alignment of the THz emitter [140].
While the long time-domain records presented in Chapter 5 were key to demon- strating the precision of the THz dual comb, they are not needed for normal mea- surements of molecular spectra. Recently, it has been shown that the full resolution of a Fourier transform infrared comb spectrometer can be extracted by detecting a single time domain ‘center-burst’ synchronized to the comb repetition rate [190].
For the ASOPS spectrometer, this is equivalent to detecting a single THz pulse record with the digitizer sample clock and total record length synchronized to the repetition rate offset frequency of the lasers. Interestingly, the digitizer does not need to be synchronized to the repetition rate of the lasers themselves, due to the zero phase CEP of the THz comb. If the CEP phase were non-zero, however, the digitizer would need to be synchronized to the laser repetition rate. Initial data has been collected on the THz comb instrument to confirm this functionality. Overall, this detection scheme will greatly simplify the comb spectrometer, and filter out low frequency sources of noise in the measurements.
Once the THz frequency comb spectrometer has been fully optimized in sensitivity and bandwidth, we hope to combine it with a free jet expansion for studies of molecular clusters. The analysis of vibration-rotational tunneling spectra of clusters can be challenging. For a particular set of beam conditions, multiple cluster species are often present, each with hundreds to thousands of rovibrational transitions in the
THz range. The assignment of pure rotational transitions in the microwave region are generally easier, however, because there are many fewer eigenstates to consider.
To aid in the THz assignment process, we propose a MW-THz double resonance instrument, as shown in Fig. 9.1. In this approach, a continuous wave microwave source is used to pump pure rotational transitions already assigned in the CP-FTMW spectrometer. For each microwave pump frequency, the THz spectrum is collected with the frequency comb, and modulations in peak intensities are monitored. This will allow specific clusters to be ‘tagged’ with the microwave source, and greatly assist in the quantum number assignments of a THz transition. For nonpolar and dynamically averaged symmetric clusters that do not have pure rotational transitions (e.g. water trimer, tetramer, and pentamer), the microwave source could be replaced with a mid-infrared laser for IR-THz double resonance tagging.
In addition to the cluster-related instruments, we also built a 2D THz-THz-Raman spectrometer. In its present form, the instrument is capable of collecting the 2D responses of liquids and amorphous solids that are transparent to 800 nm light.
Data acquisition on this instrument is quite slow, usually requiring 10 hours or more of integration to reach a sufficient signal-to-noise ratio. With the addition of a single-shot detection system, we expect an acquisition rate increase of∼20×[172].
Furthermore, wavefront shaping optics on the near-infrared pump beams and larger aperture DSTMS THz emitters will increase the THz field strength at the sample by at least 10×[173]. Due to thequadraticsignal scaling as a function of the THz field, this will increase the signal-to-noise ratio of the experiment by ∼100×. The field strength could be further improved with a second stage Ti:Sapphire amplifier (e.g.
[173]), which would increase the near-infrared pump and corresponding THz pulse energy, but lower the laser and data acquisition repetition rate. This is advantageous for 2D-TTR, though, as the signal-to-noise ratio only scales as thesquare rootof the data acquisition rate. With sufficient pulse energy and tunability (e.g. [191, 192]) the experiment could also be used for studies of vibrational ladder climbing and the coherent control of liquids and clusters.
We foresee many promising science targets with the 2D THz-THz-Raman spectrom- eter in the coming years. So far, the instrument has revealed vibrational coupling in pure halomethane liquids. To further disentangle the intra- and intermolecular contributions to the 2D spectra, measurements of binary mixtures would be es- pecially informative. Nonadditive and additive features in the spectra should be indicative of inter- and intra-molecular coupling, respectively [30]. With the laser
Figure 9.2: The TKE response of CS2measured with a two-color plasma filamen- tation THz source [193].
systems currently running in the Blake group, it should also be possible to align a 1D-OKE and 2D Raman-THz-THz spectrometer. A 2D THz-THz-THz instrument for waiting time measurements would also be invaluable, but more technically chal- lenging. These data, along with temperature controlled studies, will provide a more detailed description of the off-diagonal features in the 2D-TTR spectra observed in this thesis.
A central goal of the 2D THz-THz-Raman project is to measure hydrogen-bonded liquids. The ∼8 THz bandwidth of the instrument covers both the bending and stretching motions of hydrogen bonds and could be extended to 15 THz with HMQ- TMS organic crystal emitters [191]. However, the multi-cycle temporal profile of the THz pulses generated with these organic crystal emitters limits the time resolution.
In the 2D Raman-THz-THz response of water, for example, most of the dynamics occur in the first 400 fs after excitation [11]. A possible alternative to DSTMS is two-color plasma filamentation in air, which can produce THz pulses near the∼50 fs transform limit of the OPA [194]. We performed initial TKE measurements with
a plasma source on liquid CS2, and found that the signal-to-noise ratio was >10× lower than with DSTMS (Fig. 9.2). Electro-optic detection of THz pulses from both sources confirmed that the plasma peak THz electric field was∼3-4×smaller than that of DSTMS. Nevertheless, with proper optimization of the plasma source and THz focusing, it should be possible to reach 4 MV/cm with our laser system [194], which would be ideal for 2D-TTR spectroscopy.
A possible first target in 2D-TTR studies of hydrogen-bonding is acetic acid, which is thought to form dimers in the liquid phase. Previous OKE work on this species has revealed long-lived hydrogen bond coherences at room temperature that will be easily observed in a 2D-TTR spectrum [195]. Binary mixtures of alcohols and water are another intriguing target, due to the anomalous thermodynamic properties discussed in Chapter 3 [73]. And more long-term, we hope to measure the water of hydration around biopolymers that governs their reactivity and macromolecular structure [3].