IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 38, NO. 1, JANUARY 2002 31 Two-Photon Absorption Coefficients of Several Liquids at 264 nm Adrian Dragomir, John G. McInerney, Senior Member, IEEE, David N. Nikogosyan, and Albert A. Ruth Abstract—The nonlinear absorption of water (H O), heavy water (D O), ethanol, methanol, hexane, cyclohexane, 1, 2-dichloroethane, and chloroform at 264 nm was studied using femtosecond laser pulses. The two-photon absorption coefficients for these liquids were found to be between (34 3) 10 cm/W and (95 11) 10 cm/W. Index Terms—Femtosecond UV pulses, liquids, nonlinear trans- mission, two-photon absorption coefficient, two-photon absorption cross-section. I. INTRODUCTION L ASER-INDUCED UV two-photon absorption in water was first discovered in 1980 [1], [2]. With the rapid development of high-intensity UV laser systems and their applications in chemical and biological sciences, investigations of two-photon absorption (TPA) in liquids have become in- creasingly important. Over the last two decades, TPA has been widely used for ionization and/or dissociation of neat water, heavy water, methanol, and ethanol as well as for triggering various photophysical processes from highly excited states, such as electron solvation and geminate recombination [3]–[8]. However, only very few quantitative measurements of TPA coefficients in water [9], [10] and ethanol [11] have been performed so far. Recently, we showed that using femtosecond pulses at 264 nm from a newly developed laser system, TPA coefficients in fused silica and crystalline quartz can be measured with high accuracy [12]. In the present paper, measurements of TPA coefficients in a variety of eight liquids, i.e., water (H O), heavy water (D O), methanol, ethanol, hexane, cyclohexane, 1, 2-dichloroethane, and chloroform, are reported. II. EXPERIMENTAL SETUP The experimental setup is shown schematically in Fig. 1. The laser system 1 emitted single femtosecond pulses (about 200 fs each) with a repetition rate of 27 Hz at 264 nm (4th harmonic of Nd : glass laser radiation). The sample liquid was placed in a Suprasil cuvette with an optical pathlength of 0.2 cm and a Manuscript received July 10, 2001; revised October 1, 2001. A. Dragomir, J. G. McInerney, and D. N. Nikogosyan are with the Department of Physics, National University of Ireland, University College Cork, Cork, Ire- land (e-mail: niko@physics.ucc.ie). A. A. Ruth is with the Department of Chemistry, National University of Ire- land, University College Cork, Cork, Ireland. Publisher Item Identifier S 0018-9197(02)00176-8. 1 Twinkle, Light Conversion Ltd. total thickness of 0.5 cm. A fused silica lens with a long focal length (454 mm) was used to form a converging laser beam in order to vary the incident intensity between 5 and 30 GW/cm , depending on the position of the sample in the beam. Care was taken to keep the sample cuvette in a region between the lens and the focal point well away from the latter, where self-focusing is entirely negligible (see [12]). Two pyroelectric detectors in connection with an energy meter interfaced to a computer were used to measure the energies of the incident laser pulses and those transmitted through the sample liquid. All measurements were performed at room temperature. The following liquids were used as supplied: heavy water (Merck, 99.8%), methanol (Fluka, 99.8%), absolute ethanol (96%), hexane (Rathburn Chemicals, 95%), cyclohexane (Wardle, 99%), 1, 2-dichlorethane (Sigma Aldrich, 99.8%), and chloroform (Romil Pure Chemicals, 99.8%). The doubly distilled water was prepared in the Chemistry Department of University College Cork. The linear absorption of the samples at nm was mea- sured using the spectrophotometer (HP8453). The transmittance through the filled cuvette is determined by the expression (1) where is the linear absorption coefficient of the liquid, is the optical pathlength in the liquid, is the reflectivity of the air-fused silica interface, and is the reflectivity of the fused silica–liquid interface. Values for and the refractive index of the cell window material were determined from direct spec- trophotometric measurements with an empty cell, yielding the values of and . The reflectivity was calculated using the Fresnel formula (2) where is the refractive index of the sample liquid. The refractive indices for all liquids at nm were extrapolated from published dispersion data [13]. In Table I, the values for , , and the experimental linear absorption coefficients are listed for the eight liquids investigated. The value of either of the liquids studied is much larger than that of Suprasil fused silica ( 0.001 cm ); therefore, the linear absorption of the cuvette windows is not accounted for in the evaluation procedure. 0018–9197/02$17.00 © 2002 IEEE