Quantum Tunneling of Hydrogen Atom in Dissociation of Photoexcited Methylamine † Ran Marom, ‡ Chen Levi, ‡ Tal Weiss, ‡ Salman Rosenwaks, ‡ Yehuda Zeiri, § Ronnie Kosloff, | and Ilana Bar ‡, * Department of Physics, Ben-Gurion UniVersity, Beer SheVa 84105, Israel, Department of Biomedical Engineering, Ben-Gurion UniVersity, Beer SheVa 84105, Israel and Department of Chemistry, NRCN, Beer-SheVa 84190, Israel, Department of Physical Chemistry and the Fritz Haber Center for Molecular Dynamics, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: December 23, 2009; ReVised Manuscript ReceiVed: January 29, 2010 The probability of hydrogen atom release, following photoexcitation of methylamine, CH 3 NH 2 , is found to increase extensively as higher vibrational states on the ﬁrst excited electronic state are accessed. This behavior is consistent with theoretical calculations, based on the probability of H atom tunneling through an energy barrier on the excited potential energy surface, implying that N-H bond breaking is dominated by quantum tunneling. I. Introduction Photodissociation of polyatomic molecules is of fundamental interest and widely studied. 1–9 Often, different parameters affect and characterize the photodissociation of a particular molecule; these may include: availability of different dissociation channels, time scales of fragment separation, initial and ﬁnal states of excitation, the intensity, polarization and energy of the used photons, and the involved potential energy surfaces (PESs) and the coupling between them as well as the presence of energy barriers along the different reaction paths. The existence of energy barriers has a major effect on the release of light atoms during photodissociation. However, their role and inﬂuence on the detailed dynamical picture is still not completely understood. One appealing system for photodissociation studies is the methylamine (CH 3 NH 2 ) molecule, which ﬁgures extensively in organic and biologic building blocks, and accordingly might have implications on our understanding of photoinduced pro- cesses in molecules containing amino moieties. Already in the 1960s, 10 it was proposed that the photodissociation of methy- lamine, following broad-band excitation in the 194-244 nm range, leads to a dominant (at least 75%) N-H bond ﬁssion, together with smaller contributions from both the C-N(<5%) and C-H(∼7.5%) bond ﬁssion channels and the H 2 (<10%) elimination pathway. In a more recent photofragment transla- tional spectroscopy (PTS) study of the 222 nm photolysis of CH 3 NH 2 , evidence for the same primary channels was found, whereas all four were found to be signiﬁcant. 11 Furthermore, H(D) Rydberg atom photofragment translational spectroscopy studies indicated that most, if not all, of the observed H(D) atoms arise as a result of N-H (N-D) bond ﬁssion, 12 consistent with the earlier estimates of the relative importance of the different fragmentation channels. 10 Similar results were also found by measuring the relative H/D atom yields resulting in our vibrationally mediated photodissociation studies. 13 For example, we found that the ∼243.1 nm dissociation of CD 3 NH 2 pre- excited with one N-H stretch quanta, corresponding to a combined (vibration + UV) excitation energy of ∼44 520 cm -1 , leads to a H/D ratio of about 15. This ratio was found to decrease as higher initial vibrational states were pre-excited. Moreover, we have found that the H/D ratio, obtained following promotion of vibrationless ground state CD 3 NH 2 molecules to analogous vibronic states (excitation energies of 41 625-45 000 cm -1 ) decreases gradually from about 28 to 14, as higher states are accessed. 14 Also, theoretical calculations suggested that the transition states on the excited PES are the lowest during N-H rupture, implying its dominant dissociation. 15 The fragments resulting from N-H bond ﬁssion were considered to arise following H atom tunneling through an early barrier on the ﬁrst excited electronic state, A ˜ , and a conical intersection (CI) between the A ˜ and ground electronic, X ˜ , states. 12,15,16 The occurrence of N-H bond dissociation via tunneling was also manifested by the large inﬂuence of the NH/ND substitution, in methylamine isotopologues, on the predissociation lifetime. 17–19 Particularly, it was found that the lifetime of the excited deuterated isotopologue, CH 3 ND 2 (8.8 ps), at its origin is about 20 times longer than that of CH 3 NH 2 (0.38 ps). 17,18 In this work we report the ﬁrst study on the dissociation probability of methylamine to release hydrogen photofragments. We take a new approach, which provides a quantitative treatment of tunneling, by comparing the experimental results with those of an analytical model and with direct dynamic calculations. The dissociation probabilities were examined for excitations to different ﬁnal vibrational states on the ﬁrst bound electronic state. It was found that the dissociation probability increases as higher vibrational states on the upper electronic state are accessed. This supports the suggestion and is consistent with our theoretical predictions, based on the probability of the H atom tunneling on the excited PES, that hydrogen release is dominated by a tunneling process. II. Methods a. Experimental Section. The experimental apparatus con- sists of a home-built time-of-ﬂight mass spectrometer (TOFMS) and two different ultraviolet (UV) tunable laser beams for excitation of the methylamine molecules to the A ˜ state and for ionization of the H photofragments, respectively. The excitation schemes employed in the experiment are described below and † Part of the “Reinhard Schinke Festschrift”. * Corresponding author. E-mail: ibar@bgu.ac.il. ‡ Department of Physics. § Department of Biomedical Engineering. | Department of Physical Chemistry. J. Phys. Chem. A 2010, 114, 9623–9627 9623 10.1021/jp912107h  2010 American Chemical Society Published on Web 02/17/2010