INFRARED SPECTROSCOPY OF SIZE-SELECTED NEUTRAL AND CATIONIC AMMONIA CLUSTERS COMBINED WITH VACUUM-ULTRAVIOLET- PHOTOIONIZATION 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 IR-UV double resonance IR UV S0S0 SnSn IE V’ Ion signal Fluorescence Infrared (IR)- ultraviolet (UV) double resonance spectroscopy by using fluorescence or multiphoton ionization (MPI) detection has been developed as a powerful technique for studies of intra- and inter-molecular geometrical structures of molecular clusters. We need a chromophore for IR-UV double resonance spectroscopy. Problem 1) no emission for fluorescence detection. 2) no suitable intermediate excited state for MPI detection. It can not be applied for species having

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. We have developed two size-selective IR spectroscopic techniques based on the VUV single-photon ionization detection. 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. VUV photoionization VUV S0S0 IE Ion signal

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

Mass spectrum of ammonia 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 The size-selected IR spectra of neutral ammonia clusters could be observed. VUV-ID-IRPDS (NH 3 ) 2 + probe (NH 3 ) 3 + probe 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 Head-to-Head Absorption at ~ 2900 cm -1 should be assigned to a part of broad shared NH stretch of Hydrogen-Transferred structure. 0 kcal/mol 17.1 kcal/mol MP2/6-31G** IRPDS-VUV-PI Scaling Factor 0.96

IRPDS-VUV-PI of ammonia tetramer cation Observed IR spectrum is in accordance with simulated one for structure 2. (NH 3 ) 4 + Structure 1 Structure 2 0 kcal/mol 11.0 kcal/mol MP2/6-31G** IRPDS-VUV-PI Scaling Factor 0.96

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 (# ) from Mext, Japan.

IRPDS-VUV-PI of ammonia trimer cation IR laser power (NH 3 ) 3 + Structure 1 Structure 2 The structure of (NH 3 ) 3 + could not be determined. 0 kcal/mol 11.7 kcal/mol MP2/6-31G** Scaling Factor 0.96

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 predissociation spectroscopy of VUV-pumped ion) IR-VUV-PI 3) (IR-VUV photoionization spectroscopies) IR spectroscopy for neutral cluster IR spectroscopy for cluster cation IR spectroscopy for neutral molecule 1) Y. Matsuda, M. Mori, M. Hachiya, A. Fujii and N. Mikami Chem. Phys. Lett (2006). 2) H. B. Fu, Y. J. Hu, and E. R. Bernstein J. Chem. Phys. 124, (2006) 3) H. K. Woo, P. Wang, K. –C. Lau, X. Xing, C. Chang, and C. Y. Ng, J. Chem. Phys (2003). IR VUV Ion signal Dissociation VUV Ion signal Dissociation IR S0S0 IE S0S0 VUV IR Ion signal S0S0 IE