Two-Photon Absorption Measurements

Series of levels of the kind discussed in connection with absorption measurements from excited states (Sect.9.2.3) can also be investigated in two-photon absorption experiments. Then only a single pulsed dye laser is needed. Amplifier stages may be used to achieve sufficient power (Sect. 

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The absorption cross sections of the Sn Si transition are determined from the kinetic analyses based on the two-photon absorption measurement. In contrast, tetraphenylporphin scarcely show the S2 state fluorescence. In order to elucidate the effect of the imino hydrogens on the radiationless transitions, deuterium isotope effect on the relaxation processes from the lowest excited singlet state of metal free porphyrins has been investigated. 

The dependence of and S2 fluorescence intensities on the sample concentration was investigated between 10 - 10 M, and a sample concentration of less than 10 M was chosen to minimize the effect of concentration quenching. Two-photon absorption measurements were carried out by the excitation by relatively low power density (<10 photons cm" pulse ) in order to avoid the saturation effect on the laser power. Under these experimental conditions, the following rate equations for the concentration of S2 molecule is derived ... 

Figure 6. Calculated values for n2( —) generated from a Kramers-Kronig transformation of the fit to the two-photon absorption measurement (-).

The EP for the 1517 cmT line is shown in Fig. 18 there is a dramatic increase in the Raman cross section of this line relative to that of the intrinsic vibrational modes of the polymer bac]dt)one with decreasing tenperature. The EP has been interpreted as arising from an electronic transition localised on the backbone which has an energy of 2.39 eV. It is possible that this is the normally forbidden I a 2 A transition (16) which has been made allowed by the symnetry breaking properties of the adsoid>ed 02 Additional evidence for this assignment comes from two photon absorption measurements at 2.34 eV (58). In this case the 1517 cm Raman line is likely to have symmetry as opposed to the usual Ag symmetry imposed by RR spectroscopy. This interpretation is consistent with the predictions of the vibrational model (19). Alternatively the 2.39 eV electronic excitation and the 1517 cmT vibrational mode may be intrinsic properties but characteristic of... 

These yielded non-resonant x ixa(3w < >,a),w) at 1.907 /tm of up to 1.2 X 10 esu. Meyer et al. [520] carried out two-photon absorption measurements on poly(2,5-dibutoxy-p-phenylene acetylene) and related polymers. Representative data of these authors are shown in Fig. 12-30. 

The first-generation dendrimer 51 was directly observed by transmission electron microscopy (TEM). The TEM image showed that the dimensions of individual molecules are about 50 A, which is consistent with the calculated one [36]. Third-order NLO measurements showed a significant enhancement of two-photon absorption upon proceeding from the constituent molecules to the dendritic complex [35]. 

Kleinschmidt J, Rentsch S, Tottleben W, Wilhelmi B (1974) Measurement of strong nonlinear absorption in stilbene-chloroform solutions, explained by the superposition of two-photon absorption and one-photon absorption from the excited state. Chem Phys Lett 24 133-135... 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

At a fundamental wavelength of 1064 nm, large and anisotropic optical nonlinearity was also observed. The values of X (3) and are 4.5 x 1010 esu and 1.0 x 10 10 esu, respectively. The large values are due to the two photon resonance, because the harmonic wavelength of 355 nm is near off-resonance region. From the two-photon fluorescence measurement, we confirmed that a two-photon absorption band, which is origin of the enhancement effect, exist around 532 nm, half of the fundamental wavelength. 

Stabo-Eeg F, Lindgren M, Nilsson KPR, Inganas O, Hammarstrdm P (2007) Quantum efficiency and two-photon absorption cross-section of conjugated polyelectrolytes used for protein conformation measurements with applications on amyloid structures. Chem Phys 336 121-126... 

Doppler-free two-photon spectroscopy spect A version of Doppler free spectroscopy in which the wavelength of a transition Induced by the simultaneous absorption of two photons is measured by placing a sample In the path of a laser beam reflected on itself, so that the Doppler shifts of the Incident and reflected beams cancel. dap-lor fre tu fO,tan spek tras-ka-pe j... 

A quantitative determination of two-photon absorption cross-sections from direct measurements of chemical yield was performed by Speiser and Kimel i ) who studied the two-photon-induced photolysis of iodoform with a Q-switched ruby laser. 

The concentration of liberated iodine, as measured by its absorbance at 5000 A, is proportionalato the two-photon absorption probability, since no reactants have absorption bands at the single... 

The nuclear quadrupole resonance absorption spectra of 2,2 -bipyridine and 4,4 -bipyridine have been measured, and the photoelectron spectra of the same bipyridines and 2,2 6, 2"-terpyridine have been investigated. Two-photon absorption spectra of 2,2 -bipyridine have been taken. ... 

A technique that has been used in laboratory studies for oxides of nitrogen and shows promise for field measurements is resonance-enhanced multiphoton ionization (REMPI) (Guizard et al., 1989 Lemire et al., 1993 Simeonsson et al., 1994). For example, Akimoto and co-workers (Lee et al., 1997) have reported a REMPI system in which a (1 + 1) two-photon absorption of light at 226 nm by NO results in ionization (vide supra). They report a detection limit of 16 ppt in their laboratory studies. Other oxides of nitrogen such as NOz and HN03 can also photodissociate in the... 

Fig. 9 Two photon absorption spectra (from 2PIF measurements) of distyrylbenzene derivatives with various substitution patterns, q.3 data from [80,90] q.4 from [94] q.5 from [95,96]...

To use the two-photon fluorescence (TPF), i. e., the fluorescence excited by two-photon absorption, for the measurement of the halfwidth of ultrashort pulses from mode-locked lasers, the laser beam containing the train of pulses is split into two beams by a beam splitter (Fig. 12) which are then redirected by two mirrors Mi and M2 so that they are collinear and completely overlap in the dye cell DC which is situated in the center between the two mirrors. In the selection of the dye, it need only be remembered that it should, as far as possible, be free from absorption at the laser wavelength, have a strong absorption band at half the wavelength, and have the highest possible fluorescence quantum yield 97. Thus a... 

A typical application is the use of the (2 + 1) REMPI scheme for measuring the (v,./) distribution of H2 produced in associative desorption from a surface. When the laser is tuned to a spectroscopic transition between individual quantum states in the X -> E electronic band, resonant two-photon absorption populates the E state and this is subsequently ionized by absorption of another photon. The ion current is proportional to the number in the specific (v,./) quantum state in the ground electronic state that is involved in the spectroscopic transition. Tuning the laser to another spectroscopic feature probes another (v, J) state. Therefore, recording the ion current as the laser is scanned over the electronic band maps out the population distribution of H2(v, J) produced in the associative desorption. Ef of the (v, J) state can also often be simultaneously measured using field - free ion TOF or laser pump - probe TOF detection techniques. The (2 +1) REMPI scheme for detecting H2 is almost independent of the rotational alignment and orientation f(M) of molecules so that only relative populations of the internal states... 

Simulations of the experimental signal were performed using Equation 1 without adjustable parameters. The spectrum of the pulse and the absorption spectrum of HPTS were measured experimentally. An examination of the molecular structure of HPTS shows that it has no center of symmetry. Since parity restrictions may be relaxed in this case, the similarity between one-photon and two-photon absorption spectra is expected. The spectral phase

phase mask was the same used for the simulations. Both experimental and theoretical data were normalized such that the signal intensity is unity and the background observed is zero. The experimental data (dots) generally agree with the calculated response (continuous line) of the dyes in all pH environments (see Fig. 2). 

Figure 1. Linewidlhs of different rotational transitions in the 14q1q vibronic band of benzene measured with Doppler-free two-photon absorption. The observed strong dependence on the quantum number J of the rotational angular momentum is evidence for a rotationally dependent intramolecular coupling process. (Taken from Ref. 3.)...

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