Two-photon laser spectroscopy

Accuracy of the radiofrequency measurements of the classic 2S — 2P Lamb shift [15, 16, 23, 24, 25] is limited by the large (about 100 MHz) natural width of the 2P state, and cannot be significantly improved. New perspectives in reducing the experimental error bars of the classic 2S — 2P Lamb shift were opened with the development of the Doppler-free two-photon laser spectroscopy for measurements of the transitions between the energy levels with different principal quantum numbers. Narrow linewidth of such transi-... 

The methods of Doppler-free two-photon laser spectroscopy allow very precise comparison of the frequencies of the IS — 2S transitions in hydrogen and deuterium. The frequency difference... 

Starting with the pioneering work [52] Doppler-free two-photon laser spectroscopy was also applied for measurements of the gross structure interval in mnoninm. Experimental results [52, 53, 54, 55] are collected in Table 12.5, where the error in the first brackets is due to statistics and the second error is due to systematic effects. The highest accuracy was achieved in the latest experiment [55]... 

The method we use is Doppler free two-photon laser spectroscopy, applied to the atomic hydrogen transitions from the metastable 2S state to the Rydberg nD states (n = 8, 10, 12) /8/. 

Advantages of the two-photon laser spectroscopy are as follows high resolution, tolerance of infrared light by objects under investigation, different selection rules, and vibronic coupling. The last feature allows simultaneous accomplishment of two-photon and one-photon excitations. 

One- and two-photon laser spectroscopy of silica gel-doped fluorescent nanoparticles. Opt. Mater., 31,... 

Strickler S J, Gilbert J V and McClanaham J E 1984 Two-photon absorption spectroscopy of molecules Lasers and Applications eds H D Bist and J S Goela (New Delhi Tata McGraw-Hill) pp 351-61... 

G. Gerber By applying two-photon ionization spectroscopy with tunable femtosecond laser pulses we recorded the absorption through intermediate resonances in cluster sizes Na with n = 3,. 21. The fragmentation channels and decay pattern vary not only for different cluster sizes but also for different resonances corresponding to a particular size n. This variation of r and the fragmentation channels cannot be explained by collective type processes (jellium model with surface plasmon excitation) but rather require molecular structure type calculations and considerations. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

For instance, from the approximations introduced into the theoretical treatment of the radiation field it follows that only one-photon processes are allowed. However, very intense radiation fields, especially those produced by lasers, can cause simultaneous absorption of two photons, thus making it possible to reach molecular states that are not accessible from the ground state via one-photon absorption. Quite often, the only other evidence for the existence of these states is indirect, and two-photon absorption spectroscopy is complementary to conventional one-photon spectroscopy. (Cf. Section 1.3.6.)... 

This is conceptually the most straightforward technique. In view of the small relative change in laser intensity, it is mainly used in cases where two different laser beams are crossed in the sample. Often one of the lasers is a high-power, fixed-frequency device attenuation is measured on the second, weaker laser appropriately termed the probe, or monitor, laser. The method was used in early days of two-photon absorption spectroscopy and applied mostly to high-density and liquid samples. Its obvious drawback is poor sensitivity, as the desired quantity is obtained as a small difference between two large numbers. 

The following estimation illustrates the possible sensitivity of resonant two-photon ionization spectroscopy (Fig. 1.36a). Let Nk be the density of excited molecules in level Ek, Pki the probability per second that a molecule in level Ek is ionized by photons from laser L2 and /la = Nin aikiS.x (1.34) the number of photons absorbed per second on the transition Ei Ek.li Rk is the total relaxation rate of level Ek, besides the ionization rate (spontaneous transitions plus collision-induced deactivation) the signal rate in counts per second for the absorption path length Ax and for incident laser photons per second under steady state conditions is ... 

Once the kinetic energy has been reduced to a few raK, the earth s gravitational field could be used to further slow the atoms in an atomic "fountain," so that the projectiles can be observed for extended periods in free fall near the turning point of their parabolic trajectories. Optical two-photon Ramsey spectroscopy of such freely falling atoms is possible with a single standing wave laser field which the atoms traverse on their way up and again on their way down (Fig. 9). >25 as in ordinary Ramsey... 

FIGURE 35 Example of high-resolution spectrum in sodium vapor obtained with two-photon Doppler-free spectroscopy. [Reproduced from Bloembergen, N., and Levenson, M. D. (1976). Doppler-free two-photon absorption spectroscopy. In High Resolution Laser Spectroscopy (K. Shimoda, ed.), p. 355, Springer, New York.l... 

The transition of the excited electronic B state of Naa is located between 600 nm and 625 nm. In order to perform real-time two-photon ionization spectroscopy in this spectral region the setup shown and described in Fig. 2.2 is used (see Sect. 2.1.1). It provides laser pulses of moderate intensities with durations of either 1.4ps or 120 fs to excite the Naa molecule to its electronic B state and subsequently to ionize the excited molecule. The bandwidth of the picosecond pulse is 40cm , while for the femtosecond pulse 260 cm is estimated. Therefore, in both cases several vibrational states of the Naa system can be excited simultaneously Figure 3.40 illustrates schematically the pump probe cycles. 

Fig.9.48. Two-photon absorption spectroscopy signals for different laser beam arrangements...

Siomos, K. and Christophorou, L. G., Laser two-photon ionization spectroscopy of molecules in liquid, Chem. Phys. Lett., 72, 43, 1980. 

Thermo-molecular beam analysis Temperature modulated DSC cfr MTDSC) Thermomicrophotometry Thermo-optical analysis Thermo-oxidative degradation Time-of-flight laser-microprobe mass spectrometry Time-of-flight mass spectrometry Time-of-flight secondary ion mass spectrometry Thermal oxyluminescence Thermal programming Two-photon absorption spectroscopy... 

The relaxation mechanisms of modified or derivative nucleobases are also very helpful to understand the peculiar photochemical properties of the canonical nucleobases. In 2012, Bakalska and Delchev compared the decay paths of cytosine and isocytosine. In 2013, Engler et focused on the differences between 9H-adenine and 9-acetyladenine by carrying out a study combining data obtained from resonant two-photon ionisation spectroscopy of the laser-desorbed molecules and DFT/MRCI... 

Lubman, D. M., and Li, Liang (1990). Resonant two-photon ionization spectroscopy of biological molecules in supersonic jets volatilized by pulsed laser desorption. In Lasers and Mass-Spectrometry (ed. D. M. Lubman), pp. 352-382. Oxford University Press, New York. 

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