Multiphoton detection

S.W. Hell, J. Soukka, P.E. Hanninen, Two- and multiphoton detection as an imaging mode and means of increasing the resolution in far-fleld light microscopy. Bioimaging 3, 65-69 (1995)... 

Multiphoton detection is a frequent souree of errors in attempts to use standard avalanche photodiodes as single photon deteetors. If the diode is not really operated in the breakdown region, a detectable output pulse is obtained only if several photons are detected within the impulse response time of the diode, with a similar results as shown in Fig. 7.62. 

Another example of a teclmique for detecting absorption of laser radiation in gaseous samples is to use multiphoton ionization with mtense pulses of light. Once a molecule has been electronically excited, the excited state may absorb one or more additional photons until it is ionized. The electrons can be measured as a current generated across the cell, or can be counted individually by an electron multiplier this can be a very sensitive technique for detecting a small number of molecules excited. 

The most widely used of these tecluiiques is resonance-enlianced multiphoton ionization (REMPI) [ ]. A schematic energy-level diagram of the most conunonly employed variant (2 + 1) of this detection scheme is illustrated in the... 

Dagdigian P J, Varley D F, Liyanage R, Gordon R J and Field R W 1996 Detection of DCI by multiphoton ionization and determination of DCI and HCI internal state distributions J. Chem. Phys. 106 10 251-62... 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Two-photon excited fluorescence detection at the single-molecule level has been demonstrated for cliromophores in cryogenic solids [60], room-temperature surfaces [61], membranes [62] and liquids [63, 64 and 65]. Altliough multiphoton excited fluorescence has been embraced witli great entluisiasm as a teclmique for botli ordinary confocal microscopy and single-molecule detection, it is not a panacea in particular, photochemical degradation in multiphoton excitation may be more severe tlian witli ordinary linear excitation, probably due to absorjDtion of more tlian tire desired number of photons from tire intense laser pulse (e.g. triplet excited state absorjDtion) [61],... 

Figure 9.31 Detection of products of multiphoton dissociation by laser-induced fluorescence...

We have also carried out preliminary experiments in which we have detected the laser desorption of ethylene, cyanogen, methanol, and benzene from the Pt(s)[7(111) x (100)] surface. These spectra are shown in Figure 9. In the experiments involving ethylene, cyanogen, and methanol only neutral species are desorbed. In the case of benzene we observe the molecular parent ion in the absence of the electron beam. We believe that this is due to resonance multiphoton ionization of the benzene by the laser after desorption (resonance multiphoton ionization of benzene is very efficient with 249 nm radiation). These spectra are in marked contrast to the results of SIMS experiments which produce a wide variety of complex metal-adsorbate cluster ions. In the case of ethylene, our experiments were performed at 140 K, and under these conditions ethylene is known to be a molecular x-bonded species on the surface. In SIMS under these conditions the predominant species is CH (15)t but in the laser desorption FTMS experiments neutral ethylene is the principal species detected at low laser power. 

Detection of Higher Order Multiphoton Fluorescence from Organic Crystals... 

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

AFID = alkali-flame ionization detection FID = flame ionization detection FPD = flame photometric detection GC = gas chromatography IGEFET = interdigitated gate electrode field-effect transistor ITMS = ion trap mass spectrometry MIMS = multiphoton ionization mass spectrometry MS = mass spectrometry... 

Syage JA, Pollard JE, Cohen RB. 1988. Ultrasensitive detection of atmospheric constituents by supersonic molecular beam, multiphoton ionization mass spectroscopy. El Segundo, CA Aerospace Corp. NTIS No. AD-A202-299. 

For ion TOF measurement a probe laser was used to ionize reaction products in the reaction zone. The (1 + F) resonance-enhanced multiphoton ionization (REMPI) method was adapted for H-atom detection. The necessary vacuum ultraviolet (VUV) radiation near 121.6 nm (for Lyman-a transition) can readily be generated by a frequency-tripling technique in a Kr cell.37 The sensitivity of this (1 +1 ) REMPI detection scheme is extremely high owing to the large absorption cross-section of Lyman-a transition,... 

The advancement of the application of lasers in combination with the molecular beam technique has made a great impact in the understanding of primary photodissociation processes. For state-specific detection of small fragments, laser-induced fluorescence, multiphoton ionization, and coherent laser scattering have provided extremely detailed information on the dynamics of photodissociation. Unfortunately, a large number of interesting... 

Luntz and co-workers have recently carried out an impressive study that follows in the spirit of the Eley-Rideal work.44 Specifically, laser-assisted recombination of N-atoms desorbing to form gas-phase N2 on Ru(0001) was investigated. Experimental measurements of state-selectively detected N2 recoiling from the surface recombination event were obtained using resonance enhanced multiphoton ionization and ion time-of-flight methods. In this way translational energy distributions of individual rovibrational states could be obtained experimentally. In addition, N2-vibrational population distributions could be derived. 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Barber, P. R., Ameer-Beg, S. M., Gilbey, J. D., Edens, R. J., Ezike, I. and Vojnovic, B. (2005). Global and pixel kinetic data analysis for FRET detection by multi-photon time-domain FLIM. In Multiphoton Microscopy in the Biomedical Sciences V.Vol. 5700. SPIE, San Jose, CA, USA, pp. 171-81. 

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