For pt.I see ibid., vol.9, no.14, p.2521 (1976). Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of H2O by electron impact for collision energies from threshold to 10 eV. Differential and integral cross sections for the excitation of the (100, 001) and 010 modes are given in absolute units. Two distinct resonance regions are observed which are superimposed on a background of direct excitation: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV. The broad enhancement of the cross sections around 6 eV is attributed to a very short-lived 2A1 state of H2O-. The threshold resonances are interpreted as quasi-bound states in the dipole field of H2O. They are expected to be of general importance in electron-polar-molecule interactions.

Crossed beam experiments have been performed to study elastic scattering and vibrational excitation in e-H2O collisions for the energy range from 1 to 10 eV. The rise of the cross sections at low energies (below 2 eV) is interpreted in terms of a direct scattering mechanism, whereas the broad enhancement of the cross sections around 6 to 9 eV is attributed to H2O- states intersecting the Franck-Condon region in this energy range. Theoretical estimates for the energies of H2O- states are not in agreement with these findings.

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Resonance Raman spectra of methotrexate, using 257 nm excitation, gives new information on its pteridine moiety. JNTRODUCTIONMethotrexate (MTX) has been used extensively as chemotherapeutic agent in the treatment of leukemia and other disorders. Its molecular site of action is the enzyme dihydrofolate reductase (DHFR) which catalyses the NADPH linked reduction of dihydrofolate to tetrahydrofolate; methotrexate acts as an inhibitor of this reaction. Numerous studies have been devoted to the mechanism of this inhibition, in particular X-ray crystallographic, electronic absorption, circular dichroism and resonance Raman studies. This last method is interesting because by suitably choosing the excitation wavelength it is possible to enhance selectively (up to lo5 times) the spectrum intensity of a given chromophore and to obtain it exclusively. Resonance Raman spectroscopy may be applied to the study of the complex DHFR-MTX, because MTX absorption spectrum exhibits some bands between 230 and 400nm, some of them being in a wavelength domain where the enzyme does not absorb. For this reason, two resonance Raman studies have already been published by using excitation radiation at 363.8 nm (preresonance) and 324 and 350.6nm.lS2 In this paper, the MTX spectra obtained with excitation radiation at 303 nm and 257 nm are presented. They show that they are likely to give information, complementary to that obtained so far. EXPERIMENTALMethotrexate was purchased from Sigma and purified by DEAE Sephadex chromatography before use.3 Electronic absorption spectra were recorded with a Cary 219 spectrophotometer. They were plotted in wave numbers and analysed into Gaussian curves with a Dupont de Nemours 310 curve analyser.The resonance Raman spectra have been obtained in the wavenumber shift range 1000-1800cm-with a Jobin Yvon Ramanor HG2S double monochromator spectrometer, specially modified for UV transmission. The concave holographic gratings are used in the second order. The accyacy of the determination of band positions is 3 cm-, and all spectra were recorded with a 5 cm-I slit width. I Author to whom all correspondence should be addressed.The 303 nm excitation wavelength was obtained by frequency doubling the 606 nm radiation provided by a Chromatix CMX4 tunable pulsed dye laser, using Rhodamine 6G as a dye (Exciton.Vo.). With such a pulsed source (the laser gives a microsecond pulse of light with 25 Hz repetition rate), a digital data acquisition and processing system has been adopted, which has been extensively described el~ewhere.~ The 257nm wavelength is the second harmonic of the 514.5 nm line provided by a 95-4 Lexel Ar+ continuous laser. The doubling system is a temperature matched ADP crystal (Inrad Corp.) and the fundamental and harmonic of the 514.5 nm line are split by a quartz prism. The Raman signal is measured using a photon counting system (ORTEC) equipped with a parallel BCD digital output, and the spectra are recorded step-by-step in a multichannel analyser. Quartz spinning cells have been used to avoid the ph...