Low energy electron driven reactions in single formic acid molecules (HCOOH) and their homogeneous clusters Isabel Martin, a Tomas Skalicky, a Judith Langer, a Hassan Abdoul-Carime, a Grzegorz Karwasz,w a Eugen Illenberger, a Michal Stano b and Stefan Matejcik b a Institut fu ¨r Chemie-Physikalische und Theoretische Chemie, Freie Universita ¨t Berlin, Takustrasse 3, D -14195 Berlin b Department of Experimental Physics, Comenius University, Mlynska dolina F2, SK-84248 Bratislava Received 8th March 2005, Accepted 13th April 2005 First published as an Advance Article on the web 25th April 2005 Low energy (0–3 eV) electron attachment to single formic acid (FA) and FA clusters is studied in crossed electron/molecular beam experiments. Single FA molecules undergo hydrogen abstraction via dissociative electron attachment (DEA) thereby forming HCOO  within a low energy resonance peaking at 1.25 eV. Experiments on the isotopomers HCOOD and DCOOH demonstrate that H/D abstraction occurs at the O–H/O–D site. In clusters, electron attachment is strongly enhanced leading to a variety of negatively charged complexes with the dimer M 2  (M  HCOOH) and its dehydrogenated form M  (M–H)  as the most abundant ones. Apart from the homologous series containing the non-dissociated (M n  ) and dehydrogenated complexes (M n1  (M–H)  , n Z 1) further products are observed indicating that electron attachment at sub-excitation energies (E1 eV) can trigger a variety of chemical reactions. Among these we detect the complex H 2 O  (M–H)  which is interpreted to arise from a reaction initiated in the cyclic hydrogen bonded dimer target. In competition to hydrogen abstraction yielding the dehydrogenated complex M  (M–H)  the abstracted hydrogen atom can react with the opposite FA molecule forming H 2 O and HCO with the polar water molecule attached to the closed shell HCOO  ion. The FA dimer can thus be used as a model system to study the response of a hydrogen bridge towards dehydrogenation in DEA. 1. Introduction Formic acid (HCOOH) as the simplest organic acid has recently been identified in the interstellar medium (ISM) 1,2 and also in the coma of the Hale–Bopp comet. 3,4 It has been speculated that it may be a key compound in the formation of molecules such as acetic acid (CH 3 COOH) or glycine (NH 2 CH 2 COOH) in the ISM. These molecules 5,6 are the simplest building blocks of biomolecules and can hence serve as model systems for the properties of larger and more complex amino acids, or proteins, e.g., with respect to their behavior during exposure to high energy radiation. It is now well accepted that reactions in biological systems induced by secondary electrons constitute an important initial step towards radiation damage. 7,8 Energy deposition in living cells by high energy quanta creates a variety of reactive intermediates. Among these, electrons are the most abundant secondary species with an initial energy distribution up to about 20 eV. 9 These ballistic electrons are present in the medium for only a short time (fs–ps), during which they are slowed down by collisions thereby initiating further ionization and excitation processes and, consequently, creating reactive species like neutral radicals, ions and electrons. At sufficiently low energies they may be captured at particular molecular sites forming negatively charged transient compounds which can dissociate. The interaction of low energy electrons with bio- logically relevant molecules (including water as the dominant compound in living tissues 10 ) is hence crucial to understand the initial molecular steps in radiation damage. The formic acid dimer (FAD) is a prototype for double hydrogen bonded organic complexes 11,12 with an enthalpy of dimerization of E14.7 kcal mol 1 . Apart from this well known cyclic form of FAD, recent infrared studies in helium nano- droplets at a temperature of 0.37 K 13 suggested an additional polar acyclic structure dominated by the long-range dipole– dipole interaction. Recent beam experiments 14 demonstrated that in isolated FA molecules the dominant reaction is dehydrogenation via the DEA process e  (E1.25 eV) þ HCOOH - HCOOH # - HCOO  þ H (1) with the maximum of the resonance located at 1.25 eV. HCOOH # represents the transient negative ion (TNI) formed in the initial Franck–Condon transition. The gas phase DEA cross section at the peak of the resonance was estimated as E2  10 22 m 2 thereby identifying FA as a comparatively weak electron scavenger. Studies on electron stimulated desorption (ESD) from na- nofilms of FA 15 showed an intense H  signal appearing within a resonant feature with the maximum at 9 eV while for obvious reasons desorption of HCOO  is completely suppressed. The H  signal can be regarded to arise from the (condensed phase) complement to the gas phase dehydrogenation reaction (1) with respect to the excess charge, viz, e  (E9 eV) þ HCOOH - HCOOH # - HCOO * þ H  (2) Both reactions, however, are induced by electrons of rather different energies and hence the involved precursor (TNI) must be of a different nature, i.e. in reaction (2) electronic excitation is involved which most likely results in further decomposition of the electronically excited neutral radical. w Permanent address: Institute of Physics, Pomeranian Pedagogical Academy, Pl-76200 Slupsk, Poland. RESEARCH PAPER PCCP www.rsc.org/pccp DOI: 10.1039/b503517a 2212 Phys. Chem. Chem. Phys., 2005, 7 , 2212–2216 This journal is & The Owner Societies 2005