Time-Resolved and Short-Pulse Laser Spectroscopies

Abstract

Despite the fact that chemists are trained to v iew m ost spectroscopic data in the frequency domain, there are some instances w here tim e-do m ain information provides greater insight. Any signal reported in the frequency domain can also be expressed in the time domain through the Fourier relationships, and we use this correspondence to advantage in Fourier transform infrared (FT-IR ) and FT-NM R (nuclear magnetic resonance) spectroscopies. Despite the Fourier relationships, for many real-world studies of molecular properties, any given spectroscopic band under investigation will be inhomogeneously broadened, m aking the connection between the time-domain response and the frequency-domain spectrum less clear than we would like it to be. In these cases, it is often necessar y to perform experimental m easurem ents in both domains to obtain a com plete understanding of the system.The focus of this article is on the strengths, limitations, and uses of time-domain and short-pulse laser spectroscopies. The capabilities of time-resolved m easurements are, of course, complementary to frequency-domain techniques, and this fact becomes clear when spectroscopies in the two domains are compared.Strengths of time-resolved and short pulse spectroscopies:1. Separation of spectroscopic phenomena (e.g., Raman and uorescence). 2. U ltrahigh sensitivity absorption m easurements. 3. Access to nonlinear spectroscopies. 4. Ability to study dynamical processes directly (e.g ., ro tatio nal diffusion, uorescence and vibrational lifetime, excitation transport). 5. M easu rem en t of ph oto ind uced transient species present at very low steady-state concentrations.Weaknesses of time-resolved and short-pulse spectroscopies:1. Identi cation of an unknown in the absence of other information. 2. Ability to provide unique ''survey'' or '' ngerprint'' information on a sample. 3. Establishment of the relationship between the experimental signal and structural information on a m olecule. 4. Ability to provide absolute, quantitative information on quantities such as concentration.The items indicated in the ''weaknesses'' list are all better suited to fre quency-dom ain spectroscopies. For example, from the acquisition of a UV-visible absorption spectrum, inform ation can (som etim es) b e gained on the identity of the chromophore, and, if the chromophore is known, it is a simple matter to quantitate its presence via Beer's law. For these reasons as well as convenience of use, frequency-domain spectroscopy is ubiquitous, and essentially all spectroscopic information presented in high school and collegiate science texts is shown in the frequency domain. The bias toward presenting inform ation in the frequency domain may be related to our ability to process spectroscopic information. Our eyes are well suited to operation in the frequency domain-a practiced eye can distinguish between laser beams with a 1 nm wavelength difference. Human detection gear for optical frequencies is less well suited to resolution of

Quantitative measurement and interpretation of optical second harmonic generation from molecular interfaces

“…1 Since Shen and his colleagues laid the theoretical foundation and demonstrated the experimental feasibility of SHG and SFG for surface studies with submonolayer level sensitivity, [2][3][4][5][6][7] surface second harmonic generation (SSHG) and SFG have been used as powerful probes for structural and spectroscopic information of the molecular interfaces and molecular thin films in the past two decades. [8][9][10][11][12][13][14][15][16][17] SSHG has also become one of the most important tools for liquid and material surface characterizations. [15][16][17][18][19][20][21][22][23][24][25][26][27] SSHG has also been used to probe molecular electronic spectroscopy of molecular interfaces.…”

“…[15][16][17][18][19][20][21][22][23][24][25][26][27] SSHG has also been used to probe molecular electronic spectroscopy of molecular interfaces. 3,[28][29][30][31] There have been a number of books or book chapters, 32,33 and review articles [9][10][11][12][13][14][15][16][17] on the fundamental theory and different aspects of the applications for surface studies with SSHG over the past twenty years. Some new developments of SHG techniques, including SHG from micro-and nano-particle surfaces, [34][35][36] second harmonic microscopy (SHM), [37][38][39] SHG chirality studies, such as SHG circular dichroism (SHG-CD), SHG linear dichroism (SHG-LD) and SHG optical rotatory dispersion (SHG-ORD) [40][41][42] have also been reviewed recently.…”