- To obtain a mechanistic understanding of the chemical processes, techniques that offer a frame-by-frame visualization of molecular structure during a reaction are of vital
importance. Numerous efforts and advances have been made in order to acquire such vivid molecular “movies”, especially in the electronic excited state. Ultrafast molecular
spectroscopy method with sufficient spatial and temporal resolutions provides such a
possibility. In particular, the recent development and implementation of femtosecond
stimulated Raman spectroscopy (FSRS) has achieved previously hidden information of the structural dynamics of many intriguing photosensitive molecular systems across
disciplines. As a technique in the mixed time-frequency domain, performing FSRS has its unique benefits in tracking vibrational dynamics, enhancing transient species, improving the signal-to-noise ratio, and so on.
In this dissertation, I will cover our experimental work on optical innovation, the unique
broadband up-converted multicolor array (BUMA) technique, the incorporation of BUMA into FSRS setup, expanding the versatility and area of application possibilities and the exploratory effort of studying metalorganic complex precursors in solution for metal oxide thin films using ultrafast spectroscopy tools including FSRS.
The convenient and economical generation of background-free, spatially dispersed BUMA signals is realized in a 0.15-mm-thick BK7 glass slide with spectral tunability from visible to near IR. Using one of the BUMA sidebands as probe pulse, the FSRS measurement yields high-quality Raman spectrum spanning a broad detection window of ca. 100—4000
cm-1, where Raman peaks in both low-frequency and high-frequency regions are well
resolved without any unwanted interference pattern.
From the first prototype, we moved forward to build a versatile and efficient wavelength
tunable FSRS setup, which includes simultaneously tunable Raman pump and probe pulses
in the visible. A spectral resolution of ~12 cm-1 is achieved in the lab. Pre-resonance
enhancement of FSRS signals of a photoacid pyranine in aqueous solution by tuning
Raman pump toward the electronic absorption peak is achieved, where the stimulated
Raman gain of the 1627 cm-1 mode is increased by over 15 times.
Moreover, the methodology of concomitantly generating two distinct sets of temporally
controllable BUMA sidebands is demonstrated in a versatile optical setup. By varying the
time delay between the two incident laser pulses with suitable crossing angles, we can
continuously tune the BUMA signals from the visible to near-IR range, which sequentially
emerge on either side of the 800 nm fundamental pulse, in both BBO crystal and BK7
glass. Ultrabroad bandwidth of ~2000 cm-1 is obtained. The pump-power-dependence of sideband signal indicates that the sideband signal goes through an intrinsic nonlinear
parametric amplification process in BBO.
Further development of the FSRS setup in the ultraviolet (UV) region has been made by
using another set of BUMA sidebands with bluer center wavelength (ca. 360—460 nm).
The Stokes spectrum of 15 mM laser dye LD390 in solution with an electronic absorption
peak at ~355 nm, was found to be >21 times stronger with a 400 nm Raman pump than
that with 550 nm Raman pump. The Raman spectrum of LD390 was collected over a ~1400
cm-1 detection window for the first time. Using the same 400 nm ps Raman pump from a
home-built second harmonic bandwidth compressor, the anti-Stokes Raman spectrum turns
out to be even stronger than the Stokes spectrum, which is attributed to different resonance
enhancement factors involving ground and electronic “vibronic” levels.
To implement our technical innovations in tunable FSRS to tackle problems in materials
science, we systematically performed a set of experiments to study dynamics of metalorganic
complexes precursors for bismuth tungsten oxide thin film. Solutions of two
precursor molecules, triphenyl bismuth (Ph3Bi) and tungsten hexacarbonyl [W(CO)6] were studied. Simultaneous generation and characterization of crystalline bismuth thin film from Ph3Bi in methanol were achieved and confirmed by detection of a coherent A1g optical phonon mode of the nascent crystalline bismuth.
Subsequently, transient absorption and tunable FSRS techniques were synergistically used to investigate the UV photolysis of tungsten hexacarbonyl in solution and the ensuing
solvent molecule binding events. After 267 nm femtosecond laser irradiation, the excited state absorption band of W(CO)6 within ~400—500 nm exhibits distinct dynamics in
methanol, tetrahydrofuran, and acetonitrile on molecular timescales. The delineated
photochemical reaction pathways on the basis of ligand nucleophilicity and solvent
reorientation time provide the rational design principles for solution precursors.