Real-Time Observations of Molecular Vibrations

The time scale of molecular vibrations is on the order of 10 —10 s. The vibrational frequency of the H2 molecule, for example, is Vvib = 

5 X 10 s TVib = 2 X 10 s, and even the heavy I2 molecule still has Tvib = 5 X 10 s. With conventional techniques one always measures a time average over many vibrational periods. 

With femtosecond pump-and-probe experiments fast motion pictures of a vibrating molecule may be obtained, and the time behavior of the wave packets of coherently excited and superimposed molecular vibrations can be mapped. This is illustrated by the following examples dealing with the dynamics of molecular multiphoton ionization and fragmentation of Na2, and its dependence on the phase of the vibrational wave packet in the intermediate state [11.139]. There are two pathways for photoionization of cold Na2 molecules in a supersonic beam (Fig. 11.63)  

The photoelectrons and ions and their kinetic energies can be measured with two time-of-flight mass spectrometers arranged into opposite directions perpendicular to the molecular and the laser beams [11.141,11.142]. 

The time scale of molecular vibrations is of the order of lO flO s. The vibrational frequency of the H2 molecule, for example, is Vyjb = 1.3-10 s - Tyib = 7.6-10 s that of the Na2 molecule is Vy = 4.5-10 s Tyib = 2-10 s and even the heavy I2 molecule still has T jb = 5 -10 s. With conventional techniques one always measures a time average over many vibrational periods. 

There are numerous other examples where pico- and femtosecond spectroscopy have been applied to problems in atomic and molecular physics. Some of them will be discussed in Sect. 12.5. 

Problem 11.1. A Pockels cell inside a laser resonator is used as Q-switch. It has a maximum transmission of 95% for the applied voltage U = 0. Which voltage U is required to prevent lasing before the gain = exp(aL) of the active medium exceeds the value = 10, when the half-wave voltage of the Pockels cell is 2 kV What is the effective amplification factor G f immediately after the opening of the Pockels cell if the total cavity losses are 30% per round trip  

The second possible competing process is the two-photon excitation of wavepackets of the v = 11-18 vibrational levels in the 2 77 state of Na2 by the pump pulse, with subsequent one-photon excitation into a doubly excited 

An interesting application is the laser induced isomerization of molecules which is shown schematically for the example of the stilben molecule in Fig. 6.102. 

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Fourier transform for different, chemically very similar halomethanes and a mixture thereof. The time-domain data in Figure 7.11 can be directly interpreted as an observation of molecular motion in real time, made possible by the compressed ultrashort pulses in the microscope. From the presence of different oscillatory patterns and beatings, it already becomes clear that the different molecules can be discriminated with high resolution. Correspondingly, the Fourier spectra in Figure 7.11 show markedly different vibrational resonances, which can also be discriminated in the ternary mixture of all components. 

Fourier transform infrared spectroscopy (FTIR) is a powerful technique to probe real-time adsorbed surface species (reactants, intermediates, products) and solution constituents due to selected molecular dipole bond vibrations induced by tuned incident radiation [100]. FTIR has been used to study the formic acid electrooxidation reaction mechanism in situ by stepping or scanning the potential where species of interest are generated, from either high potentials where the intermediate species are completely oxidized (a clean surface, >1 V vs. RHE) or low potentials where the intermediate species approaches the coverage limit (blocked surface, <0.05 V vs. RHE) [100]. The three observed reaction intermediates for formic acid electrooxidation are linearly bonded COl, bridge-bonded COb, and bridge-bonded formate (HCOOad) with vibrational bands at 2,052-2,080 cm 1,810-1,850 cm , and 1,320 cm , respectively [27, 98]. The vibration frequencies of the adsorbates are influenced by the electronic characteristics and electrochemical potential of the electrode surface. Additional peaks of lesser intensity are observed for the water adlayer and sulfate/bisulfate at the electrode interface [27, 98]. 

In this book an overview has been given of the amazing opportunities provided by femtosecond real-time spectroscopy applied to small molecules and clusters. Fascinating phenomena such as control of molecular dynamics, selective state preparation, observation of vibrational wave packets on ground state PESs, ultrafast IVR, and photodissociation with unexpected and sometimes exceptional features have been introduced. 

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