J. Phys. Chem. 1993,97, 9573-9571
9573
State to State Collisional Quenching of Vibrationally Excited NO+(v) Ions Thomas Wyttenbach and Michael T. Bowers' Department of Chemistry, Uniuersity of California, Santa Barbara, California 93106 Received: April 15, 1993; In Final Form: June 29, 1993'
Vibrational relaxation of NO+(u=1,2,3) in collisions with a broad range of neutral species has been examined.
REMPI is used to form state-selected NO+@), and a monitor ion technique is used to state selectively follow the changes in vibrational level induced by collision. The NO+(u) ions are generated in the reaction cell of an FT-ICR spectrometer which is then used to measure the parent ion and monitor ion signals. The NO+(u= 1) quenching rate constants for collision with Ar, Kr, Xe, N2, C02, CS2, CH4, CF4, CHF3, CH3F, CH3C1, CH3Br, C2H4, C3H8, and NO vary from 50.4 X (Ar) to 13 X cm3/s (C3Hs). The corresponding values for NO+(v=3) are all higher. Thequenching systems studied all follow a Au = -1 selection rule. Possiblecorrelations between quenching efficiency and the physical properties of the quencher are discussed in context with the collision dynamics.
Introduction
Detection
Trapping period
Energy is a popular term which frequently appears in newspapers and journals in all kinds of contexts. In science the physical quantity energy takes a central position. From the chemical point of view an interesting question is how energy, stored within molecular species as "internal" energy, is passed on to other species. Intramolecular processesshift the energy within the molecule among the various degrees of freedom. Eventually, this energy will end up breaking a bond (chemical reaction), being emitted as a quantum of electromagnetic radiation or dispersed over a broader regime via collision with an atom, molecule, or a larger piece of matter. In the latter case a bimolecular chemical reaction may or may not occur. In general, internal energy in molecules can be stored in vibrations, in internal rotations, and/or as electronic energy. Under low-pressure conditions, electronic energy is often efficiently coverted into vibrationalenergy before any collision occurs (the so-calledquasiequilibrium hypothesis). Under these conditions, collisioninduced vibrationalenergy transfer becomes an important process. Such collisional energy-transfer processes are the focus of this paper. Starting in the later 1960sthis topicwas addressed in numerous laboratory gas-phase studies of neutral species. For example, research on hydrogen halide lasers yielded a large body of data,' giving detailed insight into collisional vibrational quenching of molecules. Experiment and theory reached a near-quantitative level.2 In the 1980s the first work on quenching of diatomic ions was carried out.3-12 This work isvery interesting from a fundamental viewpoint since ion-neutral interaction dynamics are completely different from those occurring in neutral-neutral collisions.The ion data also have an applied component as they are needed for research involving combustion, plasma systems, and atmospheric chemistry. The problem with most of the early ion studies is that the reactant ion was not formed in a specific vibrational state, making the analysis of the data ambiguous. Some additional gas-phase work was carried out on vibrational quenching of polyatomici0n~,1%19 on cooled ions in a freejet,m and by theoretical methods on systems like 02+(u) Kr21 and Nz+(v) + He.22 The experimental method employed in this work allows a straightforward interpretation of the results. It has successfully been applied to study radiative lifetimes23 and the chemistry of vibrationally excited NO+(u)." It was found in this earlier that small halogenated alkanes are very efficient quenchers, and
+
Abstract published in Advunce ACS Abstracts, September 1, 1993.
Chirp excitation Acquisition of mass spectrum
T Y
Time
NO+(v')+ M -+ Monitor ion
I
NOt(v') formation
(b) F=l
NO+(v')
RF eiection of
I
;.I
iu ' 1
2
NO+(v'-1 ,...v,
NO+(v,)
t
,...0)
M -+ Monitor ion
Time
\
+ M + Monitor Ion Figure 1. Schematic representation of experimental events. First NO+ NO+(v,)
is formed by REMPI in its vibrational level v'; then, as time proceeds during the trappingperiod, NO+(v')decays in the presence of a quencher Q to lower vibrational states v"; finally the NO+(u) vibrational state population distribution is probed during the detection process, using a monitor M whichreactsvspecificallywithNO+(v,). Theproduct modtor ion is then detected as a peak in the mass spectrum. (a) If the monitored level is the highest level (Vm = v'), M is best introduced through a pulsed valve. (b) If intermediate levels are mapped out, M is present with constant pressure to ensure controlled conditions but yielding unwanted monitor ions during the trapping time. Thoseare ejected before population probing and mass spectrum acquisition occurs.
the question was raised whether vibrational energy was lost one quantum at a time or in larget steps. Here we would like to follow up on this topic and present the results of a systematic and thorough investigation of collisional relaxation of NO+(u=1,2,3) with a series of neutral reagents. Experimental Section
The apparatus and experimental methods employed in this work have previously been described.23-25 Briefly, experiments were carried out on a Fourier transform ion cyclotron resonance
0022-3654/93/2091-9513~04.Q0~0 0 1993 American Chemical Society
Wyttenbach and Bowers
9574 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
TABLE I: Rate Constants (k)for Vibrational Quenching of NO+( v) by Collisions with Different Neutrals. Quantities Pertaining to Some Relevant Properties of the Quenchers Are Added for Comparison: Number of Atoms (n),Reduced Mass ( p ) , Dipole Moment ( p d . and Polarizability (a)(T= 300 K)
Ar
Kr Xe
N2 c02 cs2
CH4 CF4 CHF3 CH3F
CHsCl CH3Br
C2H4 ClH8 NO
n
P (am4
PD (D)
a (A3)
1 1 1 2 3 3 5 5 5 5
17.1 22.1 24.4 14.5 17.8 21.5 10.4 22.4 21.0 15.9 18.8 22.8 14.5 17.8 15.0
0 0 0 0 0 0 0 0 1.64 1.79 1.87 1.79 0 0 0.16
1.64 2.48 4.02 1.74 2.59 8.08 2.56 2.82 2.77 2.57 4.44 5.44 4.10 6.23 1.70
5 5 6 9 2
(u = 2)
(u=l)-(u=O) 0.8f 0.5 (10) 0.4f 0.5 (5) 0.5 f 0.5 (5) 0.9f 0.5 (10) 2.1 f 1.0(20) 12 f 2 (80) 1.2f 0.5 (10) 0.6f 0.5 (10) 1.2 f 0.5 (10) 3.8 f 1.0(20) 6 f 2 (30) 7 f 2 (40) 8 f 2 (60) 13 f 2 (90) 9*2(110)
-
(u < 2)
(u = 3)
-
(u < 3)
1.9f 1.0(20) 1.9f 1.0(20) 1.3 f 1.0 (15) 3.7 f 1.0 (40) 3.5 f 1.0(30)
2.7f 1.0(20) 9 f 2 (50%)
10 f 2 (60)
Quenching efficiencies (in percent), based on ADO38collision rates, are added in parentheses. Errors given are statistical errors and uncertainties a,%!:: but relative rate constants added during data analysis. Possible systematic errors in the absolute value of k are conservatively estimated to be are much more accurate than this estimate. s
..
20
0
40 60 Time (ms)
80
100
Figure2. Time evolution of the relative NO+(v=1) signal in the presence (1.5 X l~Torr)ofdifferentquenchersQ:Xe(soliddots),CH3F(squares), and C3H8 (circles). Lines represent best fits; “no Q” represents the radiative relaxation of NO+@= 1).
(FT-ICR) mass spectrometer. Vibrationally state-selected NO+( u ) ions are formed in the center of the ICR cell by resonanceenhanced multiphoton ionization (REMPI) using an excimer pumped dye laser on a pulsed and skimmed NO beam. Ions thus formed are trapped in the ICR cell while the nonionized N O molecules are rapidly pumped away. The neutral quencher is present in the cell at constant pressure. Two types of experiments were carried out. The first is analogous to the method we used to measure radiative lifetimes of NO+(u).*3 In this experiment we employ the “monitor technique” with the monitor gas introduced via a time variable pulsedvalve. Thequencher gas, Q, is present a t a constant pressure of 1.5 X 10-6Torr in the ICR cell. The sequence of experimental events is outlined in Figure la. Quenching rate constants for
-
known24~26.27and compiled in Table 11. NO+(u) quenching by the monitor M can be neglected because M+ primarily reacts with NO+(u),rather than quenching it, and because the quencher Q is 15 times more abundant than M. Under no circumstances did the monitor gas M account for as much as 5% of the NO+(u) vibrational deactivation. Radiative lifetimes of NO+(u) are known,23 and quenching rate constants for process 1 have been determined using the monitor pulsing method (Figure la). The only unknown is the quenching mechanism: that is, is NO+( ~ 2quenched ) to u = 1 or directly to u = O? The two possible mechanisms would yield a distinctly different time dependence of the vibrational state distribution of NO+(u), which can be calculated for both cases and compared to experimental data. Figure 3 shows the calculated time evolution of NO+(u=2,1,O). NO+(u=2) is formed at time t = 0 and decays as time proceeds to the lower levels u = 1 and 0 by radiative and collisional relaxation. The quencher, in this example CHsBr, is present at a constant pressure of 1.5 X 10-6 Torr. The quenching rate constant for processes 2 and 3 are known (Table I). NO+(u=2)
NO+(u= 1)
+ CH,Br
-
NO+(u=l)
+ CH,Br
(2a)
NO+(u=O)
+ CH,Br
(2b)
+ CH,Br -.,NO+(u=O) + CH,Br
(3)
-
(1)
The u distributionsshown in parts a and b of Figure 3 are obtained when the quenching mechanisms 2a and 2b are assumed, respectively. The dashed line represents the expected monitor signal (C2H51+),employing the known (Table 11) rate constants for
are summarized in Table I. These were obtained by fitting an exponential curve to the experimental data and subtracting out the contribution due to radiative relaxation. Sample data are shown in Figure 2 for Xe, CH3F, and C3H8 quenching NO+(u=l) to NO+(u=O). When NO+ is formed with u ’ > 1, it becomes necessary to determine the time evolution of the various d’states. Hence, a different experiment is required. Well-defined conditions are required in order to be able to model the expected monitor signal correspondifng to a given vibrational state distribution of NO+( u ) for comparison with the experimental data. Consequently, it is necessary to eliminate introduction of the monitor gas via the pulsed valve. Instead, the monitor gas is added with a low and constant pressure of -lo-’ Torr. The rate constants for reaction of NO+(u=0,1,2,3) with all the monitors used are
C2H51++ NO (4) These calculated monitor curves are then compared to the experimental data (Figure 3c), which are obtained using the method depicted in Figure lb. That is, after formation of vibrationally state-selected NO+(u? the ion can react during a variable trapping time. The most important processes are radiative relaxation, quenching by Q, and reaction with M. Because of the low pressure of M, only few product ions are formed which are then ejected by applying a resonant radio frequency to the ICR cell. By that time NO+ has decayed to a distribution of vibrational states which is now probed by further reaction with M for a short time of 5-10 ms and subsequent recording of the mass spectrum. The NO+(u) probing time of 5-10 ms is short enough such that the NO+(u) state distribution
+
NO+(u’) Q
+
NO+(v”)
+Q
(u”< u’)
NO+(u) + C,H,I
-
Collisional Quenching of NO+(u) Ions
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9575
TABLE Ik Monitor Reactions Used To Map Out the Vibrational Population Distribution of NO+(v) rate constants' reaction NO+(v) levels monitored 10-9 cm3/s normalized NO+(U)+ CS2 CS2+ + N O v=3 v=3 0.73 1.oo
-
NO+(u)
+ CH31
+
CH31++ N O
u = 3,u = 2, and somev = 1
u = 3, v = 2, and v = 1
-
0.00 1.3 0.92 0.23 0.00 1.6 1.12 0.73 0.04
