Scheme of the experimental setup. The ion from the electrospay source are stored in the hexapole trap, bunched with pulsed electrodes at the exit of the hexapole. The ion bunches are then accelerated and deflected before the electrostatic trap. They enter the linear ion trap through a aperture. At the exit of the trap the ions are postaccelerated at . Parent ions can be photodissociated with , laser pulses sent either in the ion trap or in a polarized interaction box located after the postacceleration (inset). The interaction box is composed of two pairs of electrodes, and . and are set at the same potential as reached in the postacceleration region, while and are set at a potential , being a few hundred volts. and are separated by , which defines a region where the potential is higher than that of the other elements in the ion path. The neutral and ionic fragments resulting from a dissociation are detected in coincidence, the neutral species reaching a position sensitive detector (PSD) while the ions are deflected in a 45° electrostatic parallel-plate analyzer before reaching another PSD.
(Color) correlation diagrams for three groups of fragment ion masses. indicates the center of the ion detector. The two dotted lines delimit four areas corresponding to neutral and ion fragments produced inside and/or outside the interaction box: (lower left) ion inside the box and neutral outside of the box, (upper left) ion outside and neutral outside, (upper right) ion outside and neutral inside, and (lower right) ion inside and neutral inside. The trajectories of the ion and neutral fragments can be calculated with the geometry and the voltages used in the experiment. The crosses indicate the positions calculated for fragmentations occurring in the polarized box while open diamonds indicate the positions calculated for fragmentations occurring after passing the polarized box. (a) for the , 131, and 132 ions. The three spots in the lower right area indicate fast fragmentation in the polarized box. Only the fragment shows delayed fragmentation as indicated by the spot in the upper left area. Scattered points in the lower left and upper right areas correspond to false coincidences. (b) for the and 146 ions. There are two major spots for each ion, located in the upper left and upper right area, which sign a two-step fragmentation with a first neutral emitted rapidly in the polarized box (upper right area) and a second neutral emitted after the box with the final ion (upper left, open diamonds). (c) for the ion. The major spot is in the lower right area, indicating fast fragmentation events.
(Color) “” correlation diagrams for different photofragmentation channels. Horizontal and vertical axis: values of the neutral and ionic fragments measured on the PSD in millimeters represented in a square. The slope of the lines is equal to the ratio between the neutral fragment mass and the ionic fragment mass for a two-body dissociation corrected by a time factor (see text). (a) ion fragment: the dotted line has a slope corresponding to loss. (b) ion fragment: the dotted line has a slope. (c) ion fragment: the dotted line has a slope. (d) ion fragment, the correlation pattern is unclear, reflecting a two-step fragmentation. The dot-dash line would correspond to the loss of one fragment, the dotted line corresponds to a loss of , and the full line to a loss of (42). (e) correlations between the fragment ion and the second neutral fragment, emitted after the polarized box, the slope of the dotted line provides the mass of the neutral fragment emitted in the second step. (f) Correlation between the first neutral fragment emitted in the polarized box and the center of mass of the ion and second neutral, the slope of the dotted line is consistent with the two-step fragmentation mechanism and . (g) ion fragment in coincidence with a neutral: (dotted line) slope , CO loss; (dashed line) slope , loss; dot-dash line: slope , loss. (h) correlation between the ion and the more scattered neutrals provides the second fragmentation step by the slope of the dotted line . (i) Correlation diagram corresponding to the ejection of the less scattered neutrals associated with the center of mass of the ion (159) and second neutral, giving a slope. The overall fragmentation is , loss of water, followed by , loss of CO.
KER distributions for different fragmentation channels. Full lines: experiments; dashed lines: simulations with the RRKM model. (a) , (b) , (c) , (d) , and (e) . The KER is usually small and agrees with the statistical calculation except for the secondary fragmentation step of the two-step fragmentation.
Scheme of the relative energies for various dissociation channels. The dissociation energies were calculated as the sum of the ionic and neutral energies by DFT calculations at the (U)B3LYP/6-31G(d) level of the energy-optimized structures of the fragments using the GAUSSIAN 98 program package (Ref. 21). The zero point energy (ZPE) correction is included into the fragment energies. The energy origin corresponds to the most stable isomeric structure. The energy available for dissociation in the present experiment is (photon energy plus thermal energy of the parent ion).
Relative populations of the ionic channels. Lower panel: all fragmentation events occurring between 0 and after the laser pulse. Upper panel: fast fragmentation events inside the polarized box (in less than ). The peaks corresponding to and 146 are only seen in the bottom spectrum and therefore correspond to delayed fragmentations while all other ionic fragments are produced very quickly.
(Color online) Photodissociation of the ion in the electrostatic ion trap. (a) Time of flight spectrum of the neutrals escaping the electrostatic trap. Before , the trapped ion decays slowly through electron capture or collision induced dissociation with the background gas. The laser is shot at and the photofragmentation produces a sharp increase of the neutral peaks, superimposed on the slowly decaying spectrum. (b) Details of the neutral time of flight spectrum at after the laser pulse. Full line: laser on, dashed line: laser off. The major peak, present with and without the laser, corresponds to neutrals coming from the trapped ion, the smaller peak at shorter times is only present when the laser is on and corresponds to neutrals coming from a trapped fragment issued from the photofragmentation. (c) Decay curve obtained in subtracting the laser off spectrum from the laser on spectrum. The short time component corresponds to the fast photofragmentation events issued from the and 204 ion, while the longer component corresponds to the unimolecular decay of the fragment ion via unimolecular fragmentation and collisions with the residual background gas.
(Color online) Scheme of the suggested mechanisms for the various dissociation pathways, depending on the “assumed” location of the electron after laser excitation [for a more accurate description of the excited states see recent ab initio calculations (Refs. 13, 22, and 27)]. Site a corresponds to the transition where the excited electron stays on the indole ring. This state may decay through proton transfer to the carbon and further H atom transfer to give the and 132 fragment. Site b corresponds to an electron transfer to the rydberg orbital located on the terminal amino group, resulting in a competition between H atom loss to produce the radical cation that subsequently decays to and loss (possibly through internal conversion) followed by secondary fragmentations to and 144. Site c corresponds to an electron transfer to the of the amino-acid part that will favor a proton transfer from the amino group to the acidic group and lead to (successive loss of and CO) and .
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