Published: May 25, 2011 r2011 American Chemical Society 1421 dx.doi.org/10.1021/jz200590m | J. Phys. Chem. Lett. 2011, 2, 1421–1425 LETTER pubs.acs.org/JPCL Experimental Signatures for a Resonance-Mediated Reaction of Bend-Excited CD 4 (v b = 1) with Fluorine Atoms Fengyan Wang † and Kopin Liu* ,†,‡,§ † Institute of Atomic and Molecular Sciences (IAMS), Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 10617 ‡ Department of Chemistry, National Tsing Hua University, Hsinchu 300, Taiwan § Department of Physics, National Taiwan University, Taipei, Taiwan 10617 M ode- and bond-selective chemistry is a fascinating subject in chemical dynamics. 15 The pioneering works by Zare’s and Crim’s groups 1,2 clearly demonstrated preferential cleavage of a vibrationally excited bond and retention of energy initially deposited in a nonreacting bond during the reaction. An intui- tively appealing picture also emerges to rationalize the observa- tion: Exciting vibrations that resemble the motion along the reaction coordinate can potentially promote the reactivity. This picture is painted from a series of studies on benchmark reactions of Cl þ methane (and isotopomers) 15 and H/Cl þ HOD, 69 in which either the CH or OH stretching mode of reactants was activated. More detailed investigations of reactions of CH stretch-excited methane with Cl, 1013 F, 14 and O( 3 P) 15 atoms revealed that the actual picture can be more subtle and compli- cated. Nonetheless, the above rudimentary concept remains a reasonable picture for guiding our thinking. Compared to the stretch-excited reactants, much less atten- tion has been paid to a bend-excited reaction. Unlike the stretching mode excitation that can be approximately regarded as an isolated, local oscillator, the bending mode excitation always involves nonlocalized, concerted motions of three or more atoms and it does not obviously map onto the reaction coordinate. A few attempts over the past few years have been made to elucidate the effects of bending-mode excitations of methane and its isotopomers in reactions with Cl 16,17 and O( 3 P) 18 atoms. By heating the pulsed valve to thermally populate the bend-excited CD 4 in a crossed-beam experiment, it was found that compared to the ground-state reaction at the same collision energy E c , bend-excited CD 4 suppresses reactivity toward O( 3 P) atoms (a mode-specific behavior). 18 Moreover, when a bend-excited CD 4 does react with O( 3 P), the initial bending energy tends to channel into OD vibration. 18 As to the Cl þ CH 4 /CD 4 reactions, the excitation of the bending/tor- sional modes promotes reactivity, and the observed 3-fold enhancement in cross section turns out to be about the same as an equivalent amount of additional translation energy (i.e., not mode-specific). 16 Similar to the ground-state reaction, the bend- excited reaction yields mainly the ground-state product pair, 16 in sharp contrast to the above O( 3 P) reactions. Both experiments prepared bend-excited methane by thermal population, thus no bending state-selectivity. The reported bend-excited reaction cross section is therefore best viewed as the average cross section of several low-lying bend/torsion-excited states. Despite the shortcoming, the premise behind the thermal-heating scheme lies in the conjecture that the approach of attacking atoms conceivably could induce collision-mixings among those prox- imate, delocalized mode characters, thereby diminishing the state-specific differential reactivity. It is significant to note that a recent study on Cl þ CH 4 (v 4 = 1), 17 in which the one-quantum excitation of umbrella mode (v 4 = 1) of CH 4 was achieved by direct infrared (IR) laser excitation, also found a 3-fold enhance- ment in reactivity, in excellent agreement with the above results determined by the thermal approach. 16 More relevant to the present study is a very recent joint experiment-theory report on the effects of the reactant bending excitation on the integral cross sections (ICS) of the F þ CHD 3 (v b = 1) reaction. 19 The experiment was a crossed-beam study using heated pulsed valve and theory was quasi-classical trajectory (QCT) calculations based on a highly accurate ab initio potential energy surface. 20 Both experiment and theory showed that the bending excitation activates the reaction at low E c and becomes inactivated at higher E c . In addition, QCT calculations predicted some reactant bend-state-specific reactiv- ity, particularly at low E c . It remains to be seen if such bending Received: May 4, 2011 Accepted: May 23, 2011 ABSTRACT: The reactions of F þ CD 4 (v b = 0 and 1) were investigated over the collisional energy range of 14 kcal mol 1 , using a time-sliced velocity-imaging technique in a crossed-beam apparatus. Both the integral and differential cross sections are reported. Experimental evidence for signatures of quantum dynamical resonances, particularly in the differential cross section, is presented in the bend-excited (v b = 1) as well as the ground-state (v b = 0) reactions. Possible involvement of multiple resonances in the title reaction is conjectured, awaiting future theoretical confirmation. SECTION: Dynamics, Clusters, Excited States