Transcript Probing very long-lived excited electronic states of

Probing very long-lived excited electronic states of
molecular cations by mass spectrometry
Prof. Myung Soo Kim
School of chemistry and
National Creative Research Initiative for
Control of Reaction Dynamics,
Seoul National University,
Seoul 151-742, Korea
I. Introduction
A. Excited electronic states
 Involved in various processes such as photochemistry,
operation of lasers, etc.
 Difficult to probe. Information scarce.
 A frontier in physical chemistry research
For example, accurate and efficient calculation of
excited state energy is the main focus in quantum
chemistry.
 Our interest  Utilization of excited electronic states
for reaction control
B. Fate of an isolated polyatomic system prepared
in an excited electronic state
1. Nonradiative decay
Internal conversion / intersystem crossing convert the electronic
energy into vibrational energy in the ground electronic state.
2. Direct photodissociation on a repulsive state
Utilized in our previous work on reaction control via
conformation selection (Nature 415, 306 (2002)).
3. Radiative decay – fluorescence
Occurs when nonradiative decay is not efficient and
electric dipole – allowed transition is present.
C. Excited electronic states of molecular ions
LUMO
HOMO
Hole states
LUMO states

Electron ionization (EI) and VUV photoionization (PI) generate
hole states mostly.
Peaks in photoelectron spectrum  hole states.

There are more excited electronic states near the ground state of
a molecular ion than that of a neutral ( presence of hole states).
 Rapid internal conversion prevalent.
Fluorescence hardly observed for polyatomic molecular cations.

D. Theory of mass spectra
1. Quasi-equilibrium theory (QET)
1) Molecular ions in various electronic (and vibrational) states
are produced by EI (or PI).
2) Ions in excited electronic states undergo rapid internal
conversion to the ground state.  Rapid conversion of
electronic energy to vibrational energy.
3) Intramolecular vibrational redistribution (IVR) occurs rapidly
also.  Transition state theory, or,
Rice-Ramsperger-Kassel-Marcus (RRKM) theory.
 QET or RRKM – QET
Wi‡(E - E0i )
ki (E)  i
hρ (E)
2. Test
 Prepare M+ with different E.
 Measure k i or product branching ratios vs. E.
 Compare with the calculated results.
3. Results
 RRKM-QET adequate for most of the cases studied.
 Some exceptions observed.
: Mostly direct dissociation in repulsive excited states.
In several cases, dissociation in excited states which do
not undergo rapid internal conversion to the ground state
suggested.
‘Isolated electronic state’
II. Initial discovery
A. Photodissociation of benzene cation
Chopper
Prism
Argon ion laser
Laser beam
Lens
J. Chem. Phys. 113, 9532 (2000)
Electric
sector
Ion beam
Electrode
assembly
Phasesensitive
detection
Magnetic sector
Laser beam
Ion source
R1
Collision
cell
R2R3R4R5R6R7
Schematic diagram of the double
focusing mass spectrometer with
reversed geometry (VG ZAB-E) modified
for photodissociation study. The inset
shows the details of the electrode
assembly.
Ion
beam
 Observed C6H6+  C6H5+, C6H4+, C4H3+, C3H3+
at 514.5nm (2.41eV), 488.0nm (2.54eV), 357nm (3.47eV)
 Instrument can detect PD occurring within ~1 sec.
E(eV)
Electronic states
( C6H6+• )
Dissociation
( Products )
6
~2
E B2u
ktot ~ 107s-1
~ 2
D E1u
ktot ~ 104s-1
5
4
C6H6+•  C6H5+ + H•
3
2
~ 2
C A2u
~2
B E2g
Energy diagram of the benzene
molecular ion. The lowest reaction
threshold (E0) is 3.66 eV for
C6H6C6H5H. ktot denotes the
total dissociation rate constant in
the ground state calculated from
previous results.
1
0
~2
X E1g (ground state)
 For PD to be observed with the present apparatus,
photoexcited C6H6+ must have E > 5 eV
Remainder ?
 Photon energy = 2.4 ~ 3.7 eV.
Intensity
A
B
5300
5500
5700
5900
Translational energy, eV
PD-MIKE profile for the production of C4H4 from the benzene ion at
357nm obtained with 2.1kV applied on the electrode assembly.
Experimental result is shown as filled circles. Reproduction of the profile
using the rate constant distribution centered at 6.3107 s-1 obtained by
experimental data is shown as the solid curve. The positions marked A and
B are the kinetic energies of products generated at the position of
photoexcitation and after exiting the ground electrode, respectively.
10
357 nm
PD
The total RRKM dissociation rate constant of
BZ as a function of the internal energy
calculated with molecular parameters in ref. 8.
The internal energies corresponding to the
dissociation rate constants of (5.51.1)107
and (53)106 s-1 for PDs at 357 and 488.0 nm,
respectively, are marked.
488.0 nm
PD
log k, k in s
-1
8
6
4
2
4
5
6
7
8
Internal energy, eV
 Excellent RRKM – QET fitting of k is known for C6H6+ dissociation.
 From measured
k
E
PD at 357nm (3.47eV)  E=6.1 ± 0.1eV  Initial E = 2.6 ± 0.1eV
PD at 488nm (2.54eV)  E=5.5 ± 0.1eV  Initial E = 3.0 ± 0.1eV
Origin of internal E prior to photoexcitation
Most likely  vibrational energy acquired at the time of EI,
either directly or via internal conversion from an
excited electronic state.
2.6  0.1 eV for 357nm PD vs. 3.0  0.1 eV for 488nm PD ?
Experimental error?
Can we quench it by increasing benzene pressure in the ion
source, by resonant charge exchange ?
C 6 H 6 +* + C 6 H 6  C 6 H 6 * + C 6 H 6 +
PD as a function of C6H6 pressure in the ion source
Relative intensity
0.04 Torr
Pressure dependences of the precursor (BZ)
intensity (–––) and photoproduct (C4H4)
intensities at 357 (·····) and 488.0 (---) nm.
Pressure in the CI source was varied
continuously to obtain these data. Pressure was
read by an ionization gauge located below the
source. The inside source pressures estimated
at three ionization gauge readings are marked.
The scale for the precursor intensity is different
from that for photoproduct intensities.
0.013 Torr
0.09 Torr
0
-6
10
10
-5
10
-4
Ionization gauge reading, Torr
Pig/Torr
P/Torr
Zc/s-1
tR/s
Ncoll
410-6
0.0051
0.13
4.2
0.6
110-5
0.013
0.33
5.8
1.9
210-5
0.025
0.63
7.6
4.8
310-5
0.038
0.96
9.0
8.6
510-5
0.063
1.59
11.2
18
710-5
0.088
2.23
13.0
29
Ion source pressure (P), collision
frequency (Zc), source residence time (tR),
and number of collisions (Ncoll) suffered by
ions exiting the ion source at some
benzene pressures.
Quenching mechanism
 PD at 488nm efficiently quenched (by every collision)
 resonant charge exchange likely.
 PD at 357nm hardly quenched. Why?
If C6H6+ undergoing PD at 357nm is in an excited
electronic state,
C6H6 +†+ C6H6  C6H6 + C6H6+†
Population of C6H6 +† does not decrease by charge exchange.
Charge exchange ionization by benzene cation
in the ion source
 One of the ionization scheme classified as chemical
ionization (CI), a useful ionization technique in mass
spectrometry.
 Add small amount of sample (s) to reagent (R) 
Electron ionization  Initially, R+ formed mostly.
 Charge exchange ionization of S by R+
R+ + S R + S+, electron transfer
Translational & vibrational energies are not important
to drive this reaction
E  IE (S)  IE (R)
Occurs efficiently when E  0, exoergic reactions.
Relative intensity of S+ formed by charge exchange with C6H6+
 At low C6H6 pressure in the source  PD at 357nm occurs
 Possible presence of long-lived C6H6+, C6H6+†.
 At high C6H6 pressure  complete quenching of PD at 357nm
 absence of C6H6+†.
Samples
Chlorobenzene
IE (eV)
9.06
Low pressure
3.6
High pressure
3.5
Fluorobenzene
9.20
3.9
1.4
Benzonitrile
9.62
5.3
0.06
Chloropentafluorobenzene
9.72
4.7
Nitrobenzene
9.86
9.91
3.8
0.01
0.06
2.5
0.02
Ethylene
10.51
3.0
0.02
Methylene chloride
11.32
4.4
0.04
Chloroform
11.37
4.7
0.03
Carbon tetrachloride
11.47
3.4
0.06
Ethane
11.52
0.09
~0
Dichlorofluoromethane
11.75
11.98
0.05
0.04
0.16
0.01
Chlorodifluoromethane
12.20
0.09
0.05
Methane
12.51
0.24
~0
Hexafluorobenzene
1-chloro-1,1-difluoroethane
Ionization Energies and the ratios of
molecular ion intensities generated by
charge exchange ionization (CI) with
BZ and by electron ionization (EI).
CI/EI ratio
6
11.5 eV
9.243 eV
4
2
0
9
10
11
12
13
Ionization energy, eV
 C6H6+ generated at high P, fully quenched  ionizes samples
with IE < 9.2eV.
cf. IE (C6H6) = 9.243eV
 C6H6+ generated at low P  ionizes samples with IE < 11.5 eV.
~
cf. IE of C6H6 to A 2 E 2gstate of C6H6 = 11.488 eV
B. Summary
Low-lying excited electronic states of C6H6+
~
X 2 E1g
~
A 2 E 2g
IE = 9.243 eV
IE =11.488 eV
~
B2 A2u
IE = 12.3 eV
~
 A 2 E 2g has a very long lifetime, ‘isolated state’.
~
~
 A 2 E 2g X 2 E1g electric dipole – forbidden.
Internal conversion must be inefficient also.
~
 For states above A 2 E 2g ,internal conversion efficient.
(Evidence – failure to ionize S with IE > 11.5 eV by charge
exchange)
C6H6 Photoelectron Spectrum
~
X
~
A
~
~
Sharp vibrational peaks for X 2 E1g and A 2 E 2g .
III. Charge exchange ionization to detect M+†
J.Am. Soc. Mass Spectrom. 12, 1120 (2001).
1. Energetics
A+ + B  A + B+,
E ,
energy defect
E  IE (B)  RE (A )
For A+ in the ground state,
E  IE (B)  IE (A)
E > 0, endoergic
= 0, resonant
< 0, exoergic
2. Charge exchange cross section
1) Charge exchange between atomic species
Massey’s adiabatic maximum rule
Maximum cross section (max) occurs at the velocity
v ~
a E
h
For E ~ 0 , max observed v ~ 0
Otherwise, max observed at high v
2) Charge exchange involving molecular species
Exoergic charge exchange (E < 0)
 Release of E as product vibration
 Energetically nearly resonant
 large  at near thermal velocity
Endoergic charge exchange (E > 0)
 Small  at near thermal velocity. Usually keV impact
energy needed.
 Reactant vibrational energy sometimes helps to
increase , but not dramatically.
Exoergicity rule
For near thermal collision
 large when E  0
 small when E > 0
3. Instrumentation
1) Requirement
Collision cell for conventional tandem mass spectrometry
G
M 

m1 , m2 , etc.,
fragmentionsfromM
G
M

M , m1 , m2 , etc

Charge exchange
M  G  M  G 
 For charge exchange at low impact energy, M+ must be
decelerated.
 Should detect G+, which moves thermally inside the cell.
 Low yield.
2) Instrumentation
Second collision cell
EM
Ion
Electric sector
beam
Magnetic sector
Conversion
dynode
First collision cell
Conversion
dynode
First collision cell
Collision
Y-lens
Cell
Ion
Source
Repeller
3) First Cell
Ion source
Collision cell
M , m1 , G 
M+
Vs
Magnetic
analyzer
Vc
Type I ions ( formed by EI in the source) KI = eVs
Type II ions ( formed by CID in the cell) KII = e [Vs+(m1/M)(Vs-Vc)]
Type III ions ( formed from collision gas) KIII = eVc
Magnetic analyzer : m/z = B2r2e2/2K
4) Second Cell
Ion source
Magnetic analyzer
M1 , M 2 , 
Vs
Collision cell
Electrostatic analyzer
M1
Vc
 Select M1 by magnetic analyzer.
 Measure ion kinetic energy by electrostatic analyzer.
 Detect ions generated from collision gas

(
KE of type III differs from those of Type I & II)
4. Charge exchange data for C6H6+
1) Second cell
~
+
RE (C6H6+, A 2 E 2g ) = 11.488 eV
IE (CS2) =10.07 eV
E = 10.07-11.488 = -1.418 eV
77 (MID)
Intensity
II
Exoergic !
Ion signal from collision gas
observed at eVc
II
II
III
Lifetime 20s or longer.
3900
3930
3960
Translational Energy, eV
3990
2) First cell
I
I
~
I
I
RE (C6H6+, A 2 E 2 g ) = 11.488 eV
IE (CS2) =10.07 eV
IE (CH3Cl) = 11.28 eV
Exoergic !
I
Ion signals from collision gas
observed and can be identified.
I
III
I
I
I
3) Relative yield of collision gas ions vs. impact energy
When ~
A2E2g state is fully quenched
~
10
-1
IE, eV
+
.
Relative Yield, (A ) / (C6H6 )
RE ( C6H6+, X 2 E1g ) = 9.243 eV
1,3-C4H6
+
.
10
-2
CS2
CH3Cl
10
-3
10
-4
10
-5
10
-6
CH3F
CH4
0
200
400
600
800
+
+
+
+
1,3-C4H6
9.08
+
1000
Primary Ion Translational Energy, eV
CS2
10.07
CH3Cl
11.28
CH3F
12.47
CH4
12.51
~2
When A
E2g state is present
~
RE ( C6H6+, A 2 E 2g ) = 11.488 eV
-1
+
1,3-C4H6
-2
CS2
.
10
+
IE, eV
+¡¤
.
Relative Yield, (A ) / (C6H6 )
10
+
CH3Cl
+
+
1,3-C4H6
9.08
CH3F
-3
10
CH4
+
CS2
10.07
CH3Cl
11.28
-5
CH3F
12.47
-6
CH4
12.51
-4
10
10
10
0
200
400
600
800
Primary Ion Translational Energy, eV
4. Summary

Collision gas ion yield is dramatically enhanced when
the charge exchange is exoergic.

Detect charge exchange signal for various collision
gases with different IE
 Presence / absence of a very long –lived state.
Estimation of its RE.
Or, charge exchange  energy titration technique to
probe excited electronic states.
IV. Benzene derivatives
J. Chem. Phys. In press, 2002.
A. Halobenzenes
C6H6
C6H5X
6b2 (Xnp∥ character)
X
3b1
1a2
1e1g
+
-
2b1 (Xnp⊥ character)
+
6b2
2b1
np
~2
 e- removal from 3b1  (3b1)-1X
B1
~2
1a2  (1a2)-1 A
A2
-1
~
6b2  (6b2) B2 B
2
~
-1
2
2b1  (2b1) C B
1
Hole states appearing in
photoelectron spectra
C6H5Cl Photoelectron Spectrum
~
B
~
X
~
~
2
 Widths of vibrational bands of B B2 & X 2 B1 are comparable.
~
 Possibility of very long lifetime for B2 B2 of C6H5Cl+
C6H5Br Photoelectron Spectrum
~
B
~
X
~
~
2
 Widths of vibrational bands of B B2 & X 2 B1 are comparable.
~
 Possibility of very long lifetime for B2 B2of C6H5Br+
C6H5I Photoelectron Spectrum
~2
~2
 B B2 bands broader than X B1
~
 Rapid relaxation of B2 B2 of C6H5I+
C6H5F Photoelectron Spectrum
(F2p∥)-1
~
X
~
 (F2p∥)-1 bands broader than X 2 B1
 Rapid relaxation
B. Triple bonds
C6H6
C6H5CN/ C6H5CCH
CX
3b1
1e1g
1a2
6b2
2b1

 6b2  (CX∥) character
2b1  (CX⊥) character
 e- removal from 3b1 
1a2 
6b2 
~
X 2 B1
~2
A A2
~
B2 B2
Hole states appearing in
photoelectron spectra
~
X
~
B
C6H5CCH
Photoelectron Spectrum
~
X
~
B
C6H5CN
Photoelectron Spectrum
~
Sharp vibrational bands for B2 B2 states.
~
2
 Possibility of very long-lived B B2 states of
C6H5CN+, C6H5CCH+.
C. Experimental results
1) C6H5Cl+
~ ) + CH Cl  C H Cl + CH Cl+
C6H5Cl+( B
3
6 5
3
~
RE (C6H5Cl+, B ) = 11.330 eV
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.330 eV = -0.05 eV,
exoergic!
~
CH3Cl+ would be observed if B of C6H5Cl+ is very long-lived.
(a)
I C4H3
+
100
.+
.+
I C4H4
I C4H2
50
0
I C4H3
+
100
.+
I C4H2
.+
II
III CH337Cl
II
.+
II
+
III CH237Cl
+
I C4H4
.+
35
III CH3 Cl
50
35
III CH2 Cl
0
(c)
I C4H3
+
100
.+
.+
I C4H4
II
I
II
III CH Cl
.+
37
3
.+
C4H2
+
37
II/III CH2 Cl
+
III CH335Cl
50
35
III CH2 Cl
Relative Intensity
(b)
0
48
49
50
m/z
51
52
Partial mass spectrum of C6H5Cl generated by 20 eV EI
recorded under the single focusing condition with 4006 eV
acceleration energy is shown in (a). (b) and (c) are mass
spectra in the same range recorded with CH3Cl in the
collision cell floated at 3910 and 3960 V, respectively.
Type II signals at m/z 49.3 and 50.3 in (b) and at m/z 49.6
and 50.6 in (c) are due to collision-induced dissociation of
C6H5Cl+ to C4H2+ and C4H3+, respectively. The peaks at
m/z 50.6 in (b) and at m/z 50.8 in (c) are due to collisioninduced dissociation of C6H5+ to C4H3+.
2) C6H5Br+
~ ) + CH Br  C H Br + CH Br+
C6H5Br+(B
3
6 5
3
~
RE (C6H5Br+, B ) = 10.633 eV
IE (CH3Br) =10.54 eV
E = 10.54 eV - 10.633 eV = -0.093 eV,
exoergic!
~
CH3Br+ would be observed if B of C6H5Br+ is very long-lived.
100
III CH381Br
79
III CH3 Br
.+
+
+
50
81
III CH2 Br
III CH279Br
Relative Intensity
.+
0
88
90
92
94
96
m/z
Partial mass spectrum obtained under the single focusing condition
with C6H5Br and CH3Br introduced into the ion source and collision cell,
respectively. C6H5Br was ionized by 20 eV EI and acceleration energy
was 4008 eV. Collision cell was floated at 3907 V.
3) C6H5CN+
~ ) + CH Cl  C H CN + CH Cl+
C6H5CN+(B
3
6 5
3
~
RE (C6H5CN+, B ) = 11.84 eV
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.84 eV = -0.56 eV,
exoergic!
~
CH3Cl + would be observed if B of C6H5CN+ is very long-lived.
III CH335Cl
I C4H4
I C4H2
.+
II
II
II
III CH237Cl
+
II
50
37
III CH3 Cl
+
II
100
.+
.+
III CH235Cl
Relative Intensity
.+
0
47
48
49
50
51
52
m/z
Partial mass spectrum obtained under the single focusing condition with C6H5CN and
CH3Cl introduced into the ion source and collision cell, respectively. C6H5CN was
ionized by 20 eV EI and acceleration energy was 4007 eV. Collision cell was floated at
3910 V. Type II signals at m/z 49.3, 50.3, and 51.3 are due to collision-induced
dissociation of C6H5CN+ to C4H2+, C4H3+, and C4H4+, respectively. Those at m/z 49.6
and 50.6 are due to collision-induced dissociation of C6H4+ to C4H2+ and C4H3+,
respectively.
4) C6H5CCH+
~ ) + CS  C H CCH + CS +
C6H5CCH+(B
2
6 5
2
~
RE (C6H5CCH+, B ) = 10.36 eV
IE (CS2) =10.07 eV
E = 10.07 eV - 10.36 eV = -0.29 eV,
exoergic!
~
CS2+ would be observed if B of C6H5CCH+ is very long-lived.
III C32S2
50
II
I C6H4
.+
.+
32 34
III C S S
100
II
Relative Intensity
.+
0
70
72
74
76
78
m/z
Partial mass spectrum obtained under the single focusing condition with
C6H5CCH and CS2 introduced into the ion source and collision cell, respectively.
C6H5CCH was ionized by 14 eV EI and acceleration energy was 4006 eV.
Collision cell was floated at 3942 V. Type II signals at m/z 73.5 and 75.7 are due
to collision-induced dissociation of C6H5CCH+ to C6H2+ and C6H4+,
respectively.
Collision gases, their ionization energies(IE) in eV , and success / failure
to generate their ions by charge exchange with some precursor ions
Precursor ion
Collision gas
IE, eV
(CH3)2CHNH2
1,3-C4H6
(butadiene)
CS2
CH3Br
C2H5Cl
CH3Cl
C2H6
O2
Xe
CHF3
8.72
9.07
10.07
10.54
10.98
11.28
11.52
12.07
12.12
13.86
C6H5Cl+• C6H5Br+•
O
C6H5CN+•
O
C6H5CCH+• C6H5I+•
O
O
O
O
X
O
X
X
X
~
Recombination energy (X)
9.066
~
Recombination energy (B) 11.330
O
O
X
O
X
C6H5F+•
O
X
O
O
X
X
X
X
X
8.991
9.71
8.75
8.754
9.20
10.633
11.84
10.36
9.771
13.81*
~
~
~
Recombination energies of the X2B1, A2A2, and B2B2 states and the
~
oscillator strengths of the radiative transitions from the B2B2 states.
State
C6H5Cl+
C6H5Br+
C6H5I+
~2
X B1
9.066
(0.0000000)
8.991
(0.0000000)
8.754
(0.0000000)
9.71
8.75
(0.0000000) (0.0000000)
~2
A A2
9.707
(0.0000008)
9.663
(0.0000001)
9.505
(0.0000000)
10.17
9.34
(0.0000010) (0.0000004)
~2
B B2
11.330
10.633
9.771
13.236
13.381
Reaction threshold 12.356
11.891
Lowest quartet
C6H5CN+
C6H5CCH+
11.84
10.36
12.664
13.3
12.7
11.07
12.725
12.41
~ 2B is not efficient for all the cases.
 Radiative decay of B
2
~ states are not dissociative.
 B
~ state. Relaxation by
 The lowest quartet states lie ~2 eV above the B
doublet – quartet intersystem crossing would not occur.
 Internal conversion must be inefficient for the ~
B states except for
~
C H I+. For the B state of C H I+, internal conversion must be efficient.
6
5
6
5
V. Vinyl derivatives
A. Detection of Type III ions by double focusing
mass spectrometry
Type I : KI = eVS
Type II : KII = e[VC + (m2/m1)(VS - VC)]
Type III : KIII = eVC
Ion source
Collision cell
Vs
Vc
Magnetic analyzer Electrostatic analyzer
Scheme
1. Set the electrostatic analyzer (kinetic energy analyzer) to
transmit ions with kinetic energy eVc.
2. Scan the magnetic analyzer (momentum analyzer, or
mass analyzer).
Detect Type III ions only.
B. Vinyl halide
C2H4
C2H3X

X
a ( Xnp∥ character)
a ( Xnp⊥ character)
a
a
a
Xnp
~
 e- removal from a (C=C)  X2 A
~
a (Xnp∥)  A 2 A
~
a (Xnp⊥)  B2 B
Hole states appearing in
photoelectron spectra
1) Vinyl chloride
~2
 Sharp vibrational bands for A A
 Possibility of very long lifetime.
2) Vinyl bromide
~
 Sharp vibrational bands for A 2 A
 Possibility of very long lifetime.
3) Vinyl iodide
~
A
~
X
~
 Sharp vibrational bands for A 2 A
 Possibility of very long lifetime.
C. CH2=CHCN, Acrylonitrile
~
X
~
A
~
Possibility of very long lifetime for A 2 A
D. CH2=CHF, Vinyl fluoride
~
X
~
A
~
 Broad A 2 A bands
~
 Short lifetime for A 2 A
E. Experimental results
1) CH2=CHCl+
~ ) + CH Cl  CH =CHCl + CH Cl+
CH2=CHCl+(A
3
2
3
~ ) = 11.664 eV
RE (CH2=CHCl+,A
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.664 eV = -0.384 eV,
exoergic!
~
CH3Cl+ would be observed if A of CH2=CHCl+ is very long-lived.
Relative Intensity
Relative Intensity
I
(a)
I
Single – focusing mass spectrum
recorded for C2H3Cl with CH3Cl
introduced to the first cell.
I
III
I
20
30
40
50
60
III
(b)
35
70
+
CH3 Cl
III III
III
20
30
40
50
Double – focusing mass spectrum
60
70
m/z
~
A state of CH2=CHCl+ is very long-lived.
2) CH2=CHBr+
~ ) + CH Br  CH =CHBr + CH Br+
CH2=CHBr+(A
3
2
3
~ ) = 10.899 eV
RE (CH2=CHBr+, A
IE (CH3Br) =10.54 eV
E = 10.54 eV – 10.899 eV = -0.359 eV,
exoergic!
~
CH3Br+ would be observed if A of CH2=CHBr+ is very long-lived.
Relative Intenxity
III
III
I
90
Relative Intensity
III
(a)
III
I
III
92
94
III
(b)
96
79
CH3 Br
+
III
98
81
CH3 Br
+
III
III
III
90
92
94
96
98
m/z
~
A state of CH2=CHBr+ is very long-lived.
3) CH2=CHI+
~ ) + CH =C=CH  CH =CHI + CH =C=CH +
CH2=CHI+(A
2
2
2
2
2
~ ) = 10.08 eV
RE (CH2=CHI+, A
IE (allene : CH2=C=CH2) =9.69 eV
E = 9.69 eV – 10.08 eV = -0.39 eV,
exoergic!
~
CH2=C=CH2 + would be observed if A of CH2=CHI+ is very long-lived.
III
Relative Intensity
(a)
35
37
III
III
39
41
III
(b)
+
C3H4
Relative Intensity
35
43
37
III
III
39
41
43
m/z
~
A state of CH2=CHI+ is very long-lived.
4) CH2=CHCN+
~ ) + Xe  CH =CHCN + Xe+
CH2=CHCN+(A
2
~ ) = 12.36 eV
RE (CH2=CHCN+, A
IE (Xe) =12.12 eV
E = 12.12 eV – 12.36 eV = -0.24 eV,
exoergic!
~
Xe+ would be observed if A of CH2=CHCN+ is very long-lived.
III
III
(a)
Relative Intensity
III
III
III
134
136
III
III
124
126
128
130
132
140
138
140
III
III
(b)
138
Relative Intensity
III
III
III
III
III
124
126
128
130
132
134
136
m/z
~
A state of CH2=CHCN+ is very long-lived.
5) CH2=CHF+
~) + CH F CH =CHF + CH F+
CH2=CHF+( A
3
2
3
~
RE (CH2=CHF+, A ) = 13.80 eV
IE (CH3F) =12.50 eV
E = 12.50 eV – 13.80 eV = -1.3 eV,
exoergic!
~
CH3F+ would be observed if A of CH2=CHF+ is very long-lived.
(a)
Relative Intensity
I
Mass spectrum of C2H3F generated
by 20 eV EI recorded under the single
focusing condition without CH3F.
20
30
40
50
I
Relative Intensity
(b)
Mass spectrum of C2H3F generated
by 20 eV EI recorded under the single
focusing condition with CH3F.
I
I
20
30
40
50
m/z
~
A state of CH2=CHF+ is not long-lived.
Precursor ions
gas IE, eV
Collision
C2H3Cl+• C2H3Br+• C2H3I+• C2H3CN+• C2H3F+•
1,3-C4H6
(butadiene)
9.07
C3H4
(Allene)
9.692
O
O
O
O
O
CH3Br
10.54
O
O
CH3Cl
11.28
O
X
Xe
12.12
X
CH3F
12.50
Ar
15.76
X
O
X
O
X
X
X
~ 10.005
Recombination energy (X)
9.804
9.35
10.91
10.63
~
Recombination energy (A) 11.664
10.899
10.08
12.36
13.80
VI. Conclusion
1. Charge exchange ionization has been developed as a
useful technique to find very long-lived excited electronic
states of polyatomic ions and estimate their recombination
energies.
2. The following very long-lived excited electronic states
have been found.
~2
~2
+
+
A
A
C 6H 6 ,
CH2CHCl ,
A E 2g
~2
~
+
C6H5Cl ,
CH2CHBr+, A 2 A
B B2
~2
~
+
C6H5Br ,
CH2CHI+, A 2 A
B B2
~2
~2
+
+
A
C6H5CN , B B2
CH2CHCN , A
C 6H 5
CCH+,
~2
B B2
Much more than found over the past 50 years!