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Ultrafast phenomena: Experimental
Published in Guo-ping Zhang, Georg Lefkidis, Mitsuko Murakami, Wolfgang Hübner, Tomas F. George, Introduction to Ultrafast Phenomena from Femtosecond Magnetism to high-harmonic Generation, 2020
Guo-ping Zhang, Georg Lefkidis, Mitsuko Murakami, Wolfgang Hübner, Tomas F. George
Femtochemistry deals with molecular vibrations, rotations, ionization, dissociation and chemical reactions on a femtosecond time scale. This field is so broad that it needs several volumes to cover the entire topic. We choose one of the simplest systems, molecular iodine, or I2. This molecule can vibrate or rotate. Suppose the vibrational frequency is ν and its period is T = 1/ν.Figure 2.14(a) schematically shows the internuclear separation R of I2 changes in real time (in the unit of 1/ν). This is purely harmonic with a single frequency. Figure 2.14(b) depicts the rotational motion, where B is the rotational constant.Figure 2.14(c) explains how the experiment is carried out. Before laser excitation, I2 vibrates on the potential energy surface X. The x-axis denotes R in the unit of Å, and the y-axis is the potential energy measured in the unit of 1000 cm−1, or 1000 wavenumbers.
Introduction
Published in Helmut H. Telle, Ángel González Ureña, Laser Spectroscopy and Laser Imaging, 2018
Helmut H. Telle, Ángel González Ureña
Finally, the exceptional control over the time duration of the laser radiation opens up many applications that would be inaccessible to traditional light sources. While light pulses down to about 1μs are feasible, by and large, any shorter pulse duration (nano-, pico-, femto-, and attosecond) is the domain of lasers. It is in particular the realm of ultrashort pulses with tp ~ 10−15–10−18 s that has given researchers access to physical and chemical processes that could only be postulated theoretically but eluded experimental proof. For example, the full knowledge of chemical reactivity requires the full understanding of an elementary chemical reaction occurring in just one single, short time-scale event. The intermediate steps in such a reaction can be studied spectroscopically (energy regime) and dynamically (time regime), giving rise to the exciting field of femtochemistry.
Picometer Detection by Adaptive Holographic Interferometry
Published in Klaus D. Sattler, Fundamentals of PICOSCIENCE, 2013
A significant problem in femtochemistry is the difficulty in creating a significant population of excited molecules. To ensure that each molecule absorbs one excitation photon in a femtosecond pulse requires a laser intensity that is so high that many molecules experience multiphoton ionization. Therefore, it is one of the most sought techniques that can deal with a relatively low fraction of excited molecules in the presence of a majority of unexcited molecules. Fortunately, HHS is a coherent process in which emission from all molecules, both excited and unexcited, add coherently to produce the observed signal. Because of this coherence, the presence of many unexcited molecules is actually an advantage, as it permits both amplitude and phase of the emission from the small excited state population to be determined. Moreover, the coherent detection provides a high sensitivity to the phase of the radiation, which reflects the evolution of the ionization potential along the dissociation coordinate.
Spectral analyses of trans- and cis-DOCO transients via comb spectroscopy
Published in Molecular Physics, 2018
Thinh Q. Bui, P. Bryan Changala, Bryce J. Bjork, Qi Yu, Yimin Wang, John F. Stanton, Joel Bowman, Jun Ye
The use of time-resolved spectroscopy for the study of elementary reaction processes, a key driver in the fundamental understanding of chemical reaction mechanisms and molecular dynamics [1], has experienced revolutionary transformation beginning from Norrish and Porter’s seminal flash photolysis experiment to ultrafast ‘femtochemistry’ by Ahmed Zewail [2]. The development of ultrafast lasers served as a cornerstone for this transition. Taking a different path, high-resolution spectroscopy and precision measurement have motivated the development of stable lasers and frequency-domain approaches. The great merge of these two scientific paths led to the eventual development of the optical frequency comb [3]. The frequency comb possesses broad spectral bandwidth and high spectral resolution in the frequency domain, making it a suitable light source for high-resolution spectroscopy in what has been termed ‘direct frequency comb spectroscopy’ (DFCS) [4]. The versatility of DFCS has more recently been extended to studies of high-resolution spectroscopy of large molecules [5,6] and chemical kinetics [7–10]. Continuing efforts are focused towards construction of high power frequency comb sources that cover 5–10 µm for future advances in high-resolution molecular spectroscopy and dynamics [11].