J. Phys. Chem. zyxwvu 1988, 92, zyxwvu 5706-5709 Ion Cyclotron Resonance lime-of-Flight Spectroscopy. Kinetic Energy of p-Iodotokene Photodissociation Fragment Ions Robert C. Dunbar* and Gary H. Weddlet Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06, and Chemistry Department, Fairfield University, Fairfield, Connecticut 06430 (Received: February 16, 1988) A new experimental technique is described for observing kinetic energies of ions in the ICR (ion cyclotron resonance) ion trap using ion time-of-flight measurements. In this approach, a free-flight interval is established during which the electrostatic trapping well in the cell is removed, allowing ions to travel freely along the magnetic field toward the cell end plates. The fraction of ions that have sufficient velocity to escape the trap during this free-flight interval is determined by ICR measurement of the ions remaining in the trap following the free-flight period, and the average velocity of the ions is derived from repetitions of the measurement for varying free-flight intervals. The technique is conveniently applied to measuring kinetic energy releases in photodissociation reactions, since the laser pulse provides a sharp starting time and position for the free-flight period. The average kinetic energy release for the photodissociation of p-iodotoluene ions at 308 nm was measured as 110 meV, which is consistent with what is known of this system. Introduction Time-of-flight observation of the products of a molecular photodissociation is a simple and direct way to find the velocities of the fragments. The ICR ion trap is an attractive instrument for studying dissociations of ionic molecules,’ having among other advantages the capability for a high degree of translational and internal thermalization of the parent ions and convenient high- resolution mass analysis of parent and daughter ions. We describe here a combination of these two approaches to give interesting new possibilities for study of photofragmentation processes. The ICR ion trap has been used to observe the kinetic energy release of both ion-molecule reaction products* and of photo- dissociation p r o d u c t ~ . ~ The principle of these previous studies was the characterization of the ability, or inability, of the frag- mentation products to climb out of the ICR trapping well along the zyxwvutsrqpon z axis. Since the trap depth is known, measuring the fraction of fragment ions retained in the trap leads to knowledge about their velocities. This approach has had notable success but has not been useful for measuring kinetic energies much below 100 meV, because it is difficult to contain ions in a well-characterized trap shallower than this. Since most interesting fragmentations of larger ions release less kinetic energy than this, the trap-escape method has not found wide application. The ICR time-of-flight (ICR-TOF) experiment takes the al- ternative approach of removing all trapping forces from the ions during a precisely known period and measuring the fraction of ions whose velocity is sufficient for them to leave the cell during this time. The sequence of events is as in Figure 1: After ion production, the ion cloud is allowed to relax to the center of the z-direction electrostatic trapping well and to thermalize its kinetic and internal energy, through a substantial number of ion-neutral collisions. Then the ion cloud is illuminated by a short laser pulse to induce photodissociation, and at the same time the electrostatic potentials in the cell are reduced to zero, allowing free ion flight along the z direction. At the end of the free-flight period the electrostatic potential is reestablished, and those ions that still remain in the ICR cell are retrapped and subsequently measured by ICR detection. The fraction of ions that fly to the cell walls during the free-flight interval depends on the ion kinetic energy, and observation as a function of varying free-flight interval gives a time-of-flight profile of travel from the cell center to the trapping plates. Experimental Section Ion motion in the ICR cell is constrained by two mechanism^:^ zyxwvut In the x-y plane, the magnetic field allows the cyclic cyclotron ‘Author to whom correspondence should be. addressed at Case Western Reserve University. Fairfield University. 0022-3654188 12092-5706%01.50/0 and magnetron motions but prevents net radial motion of the ions (except for the slow diffusional drift driven by ion-neutral col- lisions). Along the z direction, ion motion is unconstrained by the magnetic field, but an electrostatic trapping well is established by applying a positive potential VT to the trapping plates, while (in the configuration used here) the other four plates are at dc ground. The resulting electric field has an approximate quad- rupolar shape: the trapping well along the z axis is parabolic, with depth 2VT/3 (for a cubical cell). In the 1-in. cubical cell used here, the ion flight distance is 1.27 cm, giving times of the order of tens of microseconds for unconstrained ions to strike the trapping plates. In this experiment, ions were formed and allowed to relax into the center of this trapping well, after which, as shown in Figure 1, a free-flight interval of length T was initiated. During the free-flight interval the trapping potential was dropped to zero; at the end of it, the trapping potential was restored to retrap the ions remaining in the cell. To retrap the ions, the trapping po- tential was reestablished in two steps, with the idea that ions near the edge of the cell at the end of the free-flight period might be more efficiently collected back to the cell center, with lower velocities, by more gradual buildup of the trapping potential. In practice, the stepped trapping voltage turn-on did not appear to enhance the retrapping efficiency. The experiments were done at a benzene pressure of (1-5) X zy 10” Torr, giving of the order of 100 ion-molecule collisions between the ion formation pulse and the laser pulse, which was considered to be ample both to relax the ions to the center of the z-axis trapping well and to bring them to translational thermal equilibrium. In the iodotoluene experiments, the high pressure of benzene also served to produce abundant mle 218 parent ion signal by charge-transfer ionization, even at low iodotoluene pressure (around lo-’ Torr). At this total pressure, the probability of an ion-neutral collision during a 500-ps free-flight period is negligible. To minimize stray electric field gradients in the cell during the free-flight interval, the electron collector outside the exit hole for the electron beam, normally run at +10 V, was grounded during this period. To shield the cell from stray fields from the elec- tron-beam filament, a shield electrode at ground potential was ~~ ~ (1) See, for instance: Lecture Nores in Chemistry: Ion Cyclotron Reso- nance It Hartmann, H., Wanzcek, K.-P., Eds., Springer-Verlag: New York, 1982. Dunbar, R. C. Tech. Chem. 1986.6. Dunbar, R. C. In Gas-Phase Chemistry; Bowers, M. T., Ed.; Academic: New York, 1984; Vol. 3. (2) Mauclaire, G.; Derai, R.; Fenitkin, S.; Marx, R. J. Chem. Phys. 1979, 70, 4017. Rincon, M.; Pearson, J.; Bowers, M. T. Int. zyx J. Mass Spectrom. Ion Proc. 1987, 80, 133. (3) Orth, R.; Dunbar, R. C.; Riggin, M. Chem. Phys. 1977, zyx 19, 279. (4) Dunbar, R. C ; Chen, J. H.; Hays, J. D. Inr. J. Mass Specrrom. Ion Proc. 1984, 57, 39. 0 1988 American Chemical Societv I ,