Laser two-photon ionization spectrometry of perylene and naphthacene in hexane

Abstract

The two-photon Ionization spectra of perylene and naphthacene In hexane have been measured with tunable dye lasers. They have shown structure, which can be correlated with the onephoton absorption spectra. The signal-to-noise (S/N) ratio has been evaluated as functlons of the excitatlon wavelength and the laser pulse energy, and the detection limit depended on the excitation wavelength, the laser pulse energy, and the molar absorptivity. The lowest detection limits ( S I N = 3) of perylene and naphthacene were 50 pg/mL at 383 nm and 20 pg/mL at 407 nm, respectlvely; these dye laser resuits were superior to those obtained by nitrogen-laser excitation. Table I. Lasers Used in Two-Photon Ionization Experiments wavelength, nm dye solvent 337a 360-376 PBD ethanol/toluene, 50/50 ethanol/toluene, 50/50 p-dioxane 374-394 BBQ 396-412 DPS 412-424 POPOP p-dioxane 418-426 Bis-MSB p-dioxane 426-448 C-440 ethanol 444-472 (2-460 ethanol Nitrogen laser.Laser two-photon ionization in solution is a sensitive technique for detection of polynuclear aromatic hydrocarbons (1-8), drugs (3), quinones (5, 7), and vitamins (6). Experimental conditions such as laser power, bias voltage, irradiation position, and electrode configuration have to be optimized (9), and the design of a photoionization cell is very important for sensitive detection of the photoionization signal (1,2,4,5, 7, 8).The threshold for photoionization in solution is considerably lower than the ionization potential in the gas phase due to the polarization energy of the positive ion and the electron affinity of the solvent (1 0); differences are -1-3 eV for aromatic molecules (10-13). Thus, UV-vis lasers such as a nitrogen (1-3,5-8), excimer (4), or dye (3,8,12,13) laser should be able to ionize most aromatic molecules by two-photon processes. With the use of dye lasers, the two-photon ionization spectra (wavelength dependence of the photoionization signal at a constant photon flux of the laser pulse) were obtained for 9-methylanthracene (8), pyrene (121, fluoranthene (13), and N,N,N',"-tetramethyl-p-phenylenediamine (13,14). The results show that the spectrum has structure (8,12,13) and the signal tends to be larger for a shorter wavelength (8,(12)(13)(14). Since two-photon ionization is more efficient for molecules with larger molar absorptivities from the ground to the first excited states (5,7), wavelength dependence will be valuable in choosing an optimal wavelength for the twophoton ionization detection with the highest signal-to-noise

Infrared-enhanced multiphoton ionization detection of aromatic molecules in solution

“…Sir: Laser two-photon ionization spectrometry in solution is relatively new and is a sensitive and versatile technique for the detection of organic molecules that absorb photons (1)(2)(3)(4)(5)(6)(7)(8). The photocurrent signal correlates with the one-photon absorption spectrum, but the blank signal substantially increases with decreasing excitation wavelength below about 400 nm (7, 8).…”

“…The photocurrent signal correlates with the one-photon absorption spectrum, but the blank signal substantially increases with decreasing excitation wavelength below about 400 nm (7, 8). The detection sensitivity of this technique depends on laser power (8), excitation wavelength (7,8), molar absorptivity of a molecule (4, [6][7][8], and some other experimental conditions (9). The use of an intense laser at a longer wavelength with a larger molar absorptivity will improve the detection sensitivity (8), but the improvement will be limited by saturation effects (10).…”

Applications of Strong Interactions between Photons and Molecules to Analytical Sciences

“…13,14 It is clear that there is a good correspondence between the positions of the absorption peaks and those of the ionization peaks. Furthermore, in the case of ionization spectra, there is a tendency of an increase in the signal intensity toward shorter wavelengths (vide infra).…”

Holographic Spectroscopy Diffraction from Laser-Induced Gratings

“…This form of excited-state up-pumping is common in high-power, pulsed-laser excitation of molecules. It can lead to significant upper excited-state populations and photoionization in solution (the latter process has been used for sensitive detection [25][26][27]). Despite its potential value for ionization detection and its fundamental role in pulsed-laser photochemistry, excited-state up-pumping is very difficult to study because intermediate upper states are generally very short-lived and nonfluorescent.…”