Photodissociation and photoionization of highly excited HI molecules

The spectroscopy and photochemistry of HI have been studied at excitation e nergies in the vicinity of H-atom Lyman-alpha (82259 cm(-1)) using the tech niques of 1-photon vacuum UV photodissociation, resonance-enhanced multipho ton ionization (REMPI) spectroscopy and photofragment ion imaging. REMPI sp ectra, recorded using UV photons in the energy range 27350-27750 cm(-1), ex hibit complicated structure in channels corresponding to H+, I+ and HI+ ion formation. The former two channels require absorption of a minimum of 5 UV photons, whereas HI+ X(2)Pi formation requires 4 UV photons. The spectral features are assigned to resonances at the 3-photon energy involving six st ates with Omega =0 and one state with Omega =3. Ion images recorded in the same range of UV frequencies exhibit multiple velocity components for the H + ionic fragment, and the excitation energy dependences of the ring radii r eveal two mechanisms for H+ formation. The first route is 3+1 photon excita tion to super-excited Rydberg states of neutral HI with [A(2)Sigma (+)] cor es that autoionize to high vibrational levels (v(+)greater than or equal to 15) of HI+ (X(2)Pi); subsequent 1-photon photolysis of these vibrationally excited ions makes H+. The second route is dissociation of 4-photon excite d HI to H(n=2)+I(P-2(3/2)), followed by 1-photon ionization of the H(n=2) a toms. Vacuum UV photolysis of HI at 121.6 nm combined with H-Rydberg atom p hotofragment translational spectroscopy reveals dissociation via four diffe rent channels: H(n=1) atoms are formed in association with I(5p(5), P-2(3/2 )), I(5p(5), P-2(1/2)), I(5p(4) 6s(1), P-4(5/2)) and I(5p(4) 6s(1), P-2(3/2 )). Anisotropy parameters for all these channels are positive but non-limit ing, and photolysis propensities show a marked dependence on the initial ro tational state of the HI. These observations are discussed in terms of exci tation to predissociated Rydberg or ion-pair states identified in the REMPI spectra. A previously unreported Omega =3 Rydberg state is identified via HI+ formation following multiphoton absorption resonant at the 2 UV+1 visib le photon energy. A band with origin at 68326.18(73) cm(-1), is assigned to excitation to the v'=0 level of a (3)Delta (3) state composed of a Rydberg d pi electron orbiting an HI+ X(2)Pi (3/2) core.