INFRARED SPECTROSCOPYOF SIZESELECTED NEUTRAL AND CATIONIC AMMONIACLUSTERS COMBINED

INFRARED SPECTROSCOPY　OF SIZE-SELECTED NEUTRAL AND CATIONIC AMMONIA　CLUSTERS COMBINED WITH VACUUM-ULTRAVIOLETPHOTOIONIZATION MASS SPECTROMETRY Masaki Hachiya, Yoshiyuki Matsuda, Mayumi Mori, Asuka Fujii, and Naohiko Mikami Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Japan

Introduction Infrared (IR)- ultraviolet (UV) double IE resonance spectroscopy by using fluorescence or multiphoton ionization (MPI) detection has been developed as a powerful technique for Sn studies of intra- and inter-molecular geometrical V’ structures of molecular clusters. S 0 Problem It can not be applied for species having Ion signal UV IR Fluorescence IR-UV double resonance 1) no emission for fluorescence detection. 2) no suitable intermediate excited state for MPI detection. We need a chromophore for IR-UV double resonance spectroscopy.

VUV single photon ionization This difficulty may be removed by the development of a method involving the direct ionization of neutral species by using vacuum ultraviolet (VUV) light. Advantages : 1) Can ionize cluster with single photon. 2) Softly ionize cluster with small excess energy in photoionization process by using photon of near ionization threshold energy. IE Ion signal VUV S 0 VUV photoionization We have developed two size-selective IR spectroscopic techniques based on the VUV single-photon ionization detection.

Schemes of IR predissociation spectroscopy combined with VUV ionization detection VUV-ID-IRPDS VUV-ionization detected. IR predissociation spectroscopy IR spectroscopy for neutral cluster IRPDS-VUV-PI IR predissociation spectroscopy of VUV-pumped ion IR spectroscopy for cluster cation The 118 nm light is generated by tripling 355 nm light through Xe/Ar gas mixture.

Mass spectrum of ammonia clusters @118 nm Unprotonated ammonia cluster cations are unstable species. Excess energy upon VUV photoionization is not large. No extensive cluster fragmentation. H. Shinohara, N. Nishi, and N. Washida, J. Chem. Phys. 83, 1939 (1985).

VUV-ID-IRPDS of neutral ammonia clusters (NH 3)2+ probe (NH 3)3+ probe VUV-ID-IRPDS The size-selected IR spectra of neutral ammonia clusters could be observed. The 118 nm photoionization might cause the cluster fragmentation. Change the wavelength of VUV to near ionization threshold of ammonia clusters.

IRPDS-VUV-PI of ammonia dimer cation (NH 3)2+ IR laser power Hydrogen-Transferred 0 kcal/mol MP 2/6 -31 G** Scaling Factor　0. 96 Head-to-Head 17. 1 kcal/mol IRPDS-VUV-PI Absorption at ~ 2900 cm-1 should be assigned to a part of broad shared NH stretch of Hydrogen-Transferred structure.

IRPDS-VUV-PI of ammonia tetramer cation (NH 3)4+ Structure 1 0 kcal/mol MP 2/6 -31 G** Scaling Factor　0. 96 Structure 2 11. 0 kcal/mol IRPDS-VUV-PI Observed IR spectrum is in accordance with simulated one for structure 2.

Summary 1. IR spectroscopies based on VUV photoionization are developed. 2. IR spectra for neutral ammonia clusters and unprotonated ammonia 　　 cluster cations are observed. 3. Cluster structures are determined for (NH 3)2+ and (NH 3)4+. 　　　　Acknowledge : Dr. Toshihiko Maeyama (Tohoku University) 　　Mr. Daichi Sakai (Tohoku University) This study is supported by a Grant-in-Aid for specially promoted research (#16002006) from Mext, Japan.

IRPDS-VUV-PI of ammonia trimer cation (NH 3)3+ IR laser power Structure 1 0 kcal/mol MP 2/6 -31 G** Scaling Factor 0. 96 The structure of (NH 3)3+ could not be determined. Structure 2 11. 7 kcal/mol

Experimental Setup

Generation of 118 nm Light For Single Photon Ionization

Future work VUV-ID-IRPDS : 1. To prevent cluster fragmentation upon ionization process, set energy of VUV to near ionization threshold of clusters. 2. Ammonia-Ar clusters are used. 3. To determine cluster structures and make assignment of observed IR spectra, perform ab-initio, DFT calculation. IRPDS-VUV-PI : 1. Extend wavelength of IR laser to lower energy 2. Use the tandem quadrupole mass filter-type spectrometer with VUV photoionization 3. Perform ab-initio, DFT calculation.

Schemes of IR-VUV photoionization spectroscopy VUV-ID-IRPDS 1), 2) (VUV-ionization-detected IR predissociation spectroscopy) IRPDS-VUV-PI 1) IR-VUV-PI 3) (IR predissociation spectroscopy of VUV-pumped ion) (IR-VUV photoionization spectroscopies) Dissociation Ion signal IR IE IE VUV VUV Dissociation S 0 IR IR spectroscopy for neutral cluster S 0 IR spectroscopy for cluster cation IR IR spectroscopy for neutral molecule 1) Y. Matsuda, M. Mori, M. Hachiya, A. Fujii and N. Mikami Chem. Phys. Lett. 422. 378 (2006). 2) H. B. Fu, Y. J. Hu, and E. R. Bernstein J. Chem. Phys. 124, 024302 (2006) 3) H. K. Woo, P. Wang, K. –C. Lau, X. Xing, C. Chang, and C. Y. Ng, J. Chem. Phys. 119. 9333 (2003).