probing very long-lived excited electronic states of molecular cations by mass spectrometry
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
Probing very long-lived excited electronic states of molecular cations by mass spectrometry
School of chemistry and National Creative Research Initiative for
Control of Reaction Dynamics, Seoul National University,
Seoul 151-742, Korea
Prof. Myung Soo Kim
I. Introduction
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
A. Excited electronic states
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
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.
LUMO
HOMO
Hole states LUMO states
D. Theory of mass spectra
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
1. Quasi-equilibrium theory (QET)
(E)
)E(EW(E)
‡
hρ
- k 0ii
ii
Prepare M+ with different E. Measure or product branching ratios vs. E. Compare with the calculated results.
2. Test
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’
ik
II. Initial discovery
A. Photodissociation of benzene cation
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.
Magnetic sector
Ion source
Electrode assembly
Electric sector
PM
T
Chopper
Lens
Argon ion laser
Phase-sensitive detection
Laser beamPrism
Laser beam
Collision cell
R1 R2R3 R6R4 R7R5
Ion beam
Ion beam
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.
J. Chem. Phys. 113, 9532 (2000)
For PD to be observed with the present apparatus, photoexcited C6H6
+ must have E > 5 eV
Remainder ?
Photon energy = 2.4 ~ 3.7 eV.
Energy diagram of the benzene molecular ion. The lowest reaction threshold (E0) is 3.66 eV for C6H6
C6H5
H. ktot denotes the total diss
ociation rate constant in the ground state calculated from previous results.
X 2E1g (ground state)
B 2E2g
C 2A2u
D 2E1u
E(eV)
0
2
Dissociation( Products )
~
~
~
~
E 2B2u
~
Electronic states( C6H6
+• )
ktot ~ 107s-1
ktot ~ 104s-1
C6H6+• C6H5
+ + H•
1
3
4
5
6
PD-MIKE profile for the production of C4H4 from the benzene ion at 357
nm 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 6.3107 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.
5300 5500 5700 5900
B
A
Inte
nsit
y
Translational energy, eV
Excellent RRKM – QET fitting of is known for C6H6+ dissociation.
From measured 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
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 (5.51.1)107 and (53)106 s-1 for PDs at 357 and 488.0 nm, respectively, are marked.
4 5 6 7 8
2
4
6
8
10357 nm PD
488.0 nm PD
log
k, k
in s
-1
Internal energy, eV
k
k
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 ?
C6H6 +* + C6H6
C6H6* + C6H6+
PD as a function of C6H6 pressure in the ion source
Pressure dependences of the precursor (BZ) intensity (–––) and photoproduct (C4H4
) intensit
ies 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.
Ion source pressure (P), collision frequency (Zc), source residence time (tR), and num
ber of collisions (Ncoll) suffered by ions exiti
ng the ion source at some benzene pressures.
10-6 10-5 10-4
0
0.09 Torr
0.04 Torr
0.013 Torr
Rel
ati
ve
inte
nsi
ty
Ionization gauge reading, Torr
Pig/Torr P/Torr Zc/s-1 tR/s Ncoll
410-6 0.0051 0.13 4.2 0.6
110-5 0.013 0.33 5.8 1.9
210-5 0.025 0.63 7.6 4.8
310-5 0.038 0.96 9.0 8.6
510-5 0.063 1.59 11.2 18
710-5 0.088 2.23 13.0 29
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 ele
ctronic 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
Occurs efficiently when , exoergic reactions.
(R) IE(S) IEE 0E
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+†.
Ionization Energies and the ratios of molecular ion intensities generated by charge exchange ionization (CI) with BZ and by electron ionization (EI).
High pressure Low pressure IE (eV)
1.4
3.5
Samples
Fluorobenzene
Benzonitrile
Chlorobenzene
Carbon tetrachloride
Ethane
Dichlorofluoromethane
1-chloro-1,1-difluoroethane
Chlorodifluoromethane
Methane
Ethylene
Methylene chloride
Chloroform
Chloropentafluorobenzene
Nitrobenzene
Hexafluorobenzene
9.06
9.20
11.47
11.37
11.32
10.51
9.91
9.86
9.72
9.62
12.20
11.52
11.75
11.98
12.51
3.6
3.9
5.3
4.7
3.8
2.5
3.0
4.4
4.7
3.4
0.09
0.05
0.16
0.09
0.24
0.06
0.01
0.06
0.02
0.02
0.04
0.03
0.06
~0
0.04
0.01
0.05
~0
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 state of C6H6 = 11.488 eVg22EA
~
9 10 11 12 13
0
2
4
6 9.243 eV 11.5 eV
CI/
EI
rati
o
Ionization energy, eV
B. Summary
Low-lying excited electronic states of C6H6+
g22EA
~ has a very long lifetime, ‘isolated state’.
electric dipole – forbidden. Internal conversion must be inefficient also.
For states above ,internal conversion efficient. (Evidence – failure to ionize S with IE > 11.5 eV by charge exchange)
g22EA
~g1
2EX~
g22EA
~
g22EA
~g1
2EX~
u22 AB
~
IE = 9.243 eV IE =11.488 eV IE = 12.3 eV
Sharp vibrational peaks for and .g12EX
~g2
2EA~
C6H6 Photoelectron Spectrum
X~
A~
III. Charge exchange ionization to detect M+†
1. Energetics
A+ + B A + B+, E , energy defect
For A+ in the ground state,
E > 0, endoergic
= 0, resonant
< 0, exoergic
(A) IE(B) IEE
)(A RE(B) IEE
J.Am. Soc. Mass Spectrom. 12, 1120 (2001).
2. Charge exchange cross section
1) Charge exchange between atomic species
Massey’s adiabatic maximum rule Maximum cross section (max) occurs at the velocity
For ~ 0 , max observed v ~ 0
Otherwise, max observed at high v
Eh
E a ~ v
2) Charge exchange involving molecular species
E Release of 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
Exoergic charge exchange (E < 0)
3. Instrumentation
Collision cell for conventional tandem mass spectrometry
1) Requirement
M from ionsfragment .,etc ,m ,m M 21G
M
G
cet ,m ,m ,M 21
Charge exchange GMGM
For charge exchange at low impact energy, M+ must be decelerated.
Should detect G+, which moves thermally inside the cell. Low yield.
2) Instrumentation
First collision cell
Ion
beam
Ion source
Magnetic sectorConversion
dynode
EM
Electric sector
Repeller
Ion Source
First collision cell
Conversion dynode
PM
Second collision cell
Collision Cell Y-lens
3) First Cell
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
Vs
M+Magnetic
analyzer
Vc
G ,m ,M 1
Ion source Collision cell
4) Second Cell
Vs Vc
Ion source
,M ,M 21
Magnetic analyzer Electrostatic analyzerCollision cell
Select 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)
1M
1M
4. Charge exchange data for C6H6+
1) Second cell
RE (C6H6+, ) = 11.488 eV
IE (CS2) =10.07 eV
E = 10.07-11.488 = -1.418 eV
Exoergic !Ion signal from collision gas observed at eVc
Lifetime 20s or longer.
g22EA
~
3900 3930 3960 3990
77+(MID)
II
II
II
III
Inte
nsi
ty
Translational Energy, eV
2) First cell
RE (C6H6+, ) = 11.488 eV
IE (CS2) =10.07 eV
IE (CH3Cl) = 11.28 eV
Exoergic !
Ion signals from collision gas observed and can be identified.
g22EA
~I II I
IIII
I I I
I
3) Relative yield of collision gas ions vs. impact energy
When A2E2g state is fully quenched
RE ( C6H6+, ) = 9.243 eV g1
2EX~
0 200 400 600 800 100010
-6
10-5
10-4
10-3
10-2
10-1
1,3-C4H
6
+
CS2
+
CH3Cl
+
CH3F
+
CH4
+
Rel
ativ
e Y
ield
, (A
+. ) /
(C
6H6+
. )
Primary Ion Translational Energy, eV
IE, eV
1,3-C4H6 9.08
CS2 10.07
CH3Cl 11.28
CH3F 12.47
CH4 12.51
~
0 200 400 600 80010
-6
10-5
10-4
10-3
10-2
10-1
Rel
ativ
e Y
ield
, (A
+. ) /
(C
6H6+
. )
1,3-C4H
6
+¡¤
CS2
+
CH3Cl
+
CH3F
+
CH4
+
Primary Ion Translational Energy, eV
When A2E2g state is present
RE ( C6H6+, ) = 11.488 eV g2
2EA~
IE, eV
1,3-C4H6 9.08
CS2 10.07
CH3Cl 11.28
CH3F 12.47
CH4 12.51
~
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
A. Halobenzenes
1e1g
3b1
1a2
6b2
2b1
np
C6H6 C6H5X X
e- removal from 3b1 (3b1)-1
1a2 (1a2)-1
6b2 (6b2)-1
2b1 (2b1)-1
22AA
~
22 BB
~
12BX
~
12 BC
~
Hole states appearing in photoelectron spectra
6b2 (Xnp∥ character) 2b1 (Xnp⊥ character)
++
--
J. Chem. Phys. In press, 2002.
Widths of vibrational bands of & are comparable.
Possibility of very long lifetime for of C6H5Cl+22 BB
~1
2BX~
22 BB
~
C6H5Cl Photoelectron Spectrum
X~
B~
Widths of vibrational bands of & are comparable.
Possibility of very long lifetime for of C6H5Br+2
2 BB~
12BX
~2
2 BB~
C6H5Br Photoelectron Spectrum
X~
B~
bands broader than
Rapid relaxation of of C6H5I+
22 BB
~
22 BB
~1
2BX~
C6H5I Photoelectron Spectrum
(F2p∥)-1
(F2p∥)-1 bands broader than
Rapid relaxation
12BX
~
C6H5F Photoelectron Spectrum
X~
B. Triple bonds
6b2 (CX∥) character
2b1 (CX⊥) character
e- removal from 3b1
1a2
6b2 2
2 BB~ 2
2AA~
12BX
~
1e1g
3b1
1a2
6b2
2b1
C6H6 C6H5CN/ C6H5CCH C X
Hole states appearing in photoelectron spectra
Sharp vibrational bands for states.
Possibility of very long-lived states of
C6H5CN+, C6H5CCH+.
22 BB
~
22 BB
~
B~
B~
C6H5CCH
Photoelectron Spectrum
C6H5CN
Photoelectron Spectrum
X~
X~
C. Experimental results
1) C6H5Cl+
RE (C6H5Cl+, ) = 11.330 eV
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.330 eV = -0.05 eV,
B~
~
C6H5Cl+( ) + CH3Cl C6H5Cl + CH3Cl+B~
CH3Cl+ would be observed if B of C6H5Cl+ is very long-lived.
exoergic!
Partial mass spectrum of C6H5Cl generated by 20 eV EI r
ecorded 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 colli
sion 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 collision-induced dissociation of C6H5
+ to C4H3
+.
0
50
100
+
+
+. .
(a)
C4H
4
C4H
3
C4H
2
I
I
I
0
50
100
+
+
+++
++
.. .
.
(b)
C4H
4
CH 33
7 Cl
C4H
2
CH 33
5 Cl
C4H
3
CH 23
7 Cl
CH 23
5 Cl
III
III
III
III
II II II
I
I
I
Rela
tive In
ten
sit
y
48 49 50 51 52
0
50
100
+
+
+
+
+
+
+
.
..
.
m/z
(c)
CH 23
7 Cl
II
II
C4H
4
C4H
3
C4H
2
CH 33
7 ClC
H 335 C
l
CH 23
5 Cl
II/I
II
III
III
III I
I
I
2) C6H5Br+
RE (C6H5Br+, ) = 10.633 eV
IE (CH3Br) =10.54 eV
E = 10.54 eV - 10.633 eV = -0.093 eV,
B~
~
C6H5Br+( ) + CH3Br C6H5Br + CH3Br+ B~
CH3Br+ would be observed if B of C6H5Br+ is very long-lived.
exoergic!
88 90 92 94 96
0
50
100
+
++
+
..
CH
381B
r
CH
379B
r
CH
281B
r
CH
279B
r
III
III
III
III
Rel
ativ
e In
tens
ity
m/z
Partial mass spectrum obtained under the single focusing condition with C6H5Br and CH3Br introduced into the ion source and collision cell, res
pectively. C6H5Br was ionized by 20 eV EI and acceleration energy was
4008 eV. Collision cell was floated at 3907 V.
3) C6H5CN+
RE (C6H5CN+, ) = 11.84 eV
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.84 eV = -0.56 eV,
B~
~
C6H5CN+( ) + CH3Cl C6H5CN + CH3Cl+ B~
CH3Cl + would be observed if B of C6H5CN+ is very long-lived.
exoergic!
47 48 49 50 51 52
0
50
100+
+
+
+
+
+
.
.
.
.
C4H
4
CH
337C
l
C4H
2 C
H237
Cl
CH
335C
l
CH
235C
l
IIIII
III
III
III
IIIIII
II
I
I
Rel
ativ
e In
tens
ity
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 ionize
d 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 du
e to collision-induced dissociation of C6H4+ to C4H2
+ and C4H3+, respectively.
4) C6H5CCH+
RE (C6H5CCH+, ) = 10.36 eV
IE (CS2) =10.07 eV
E = 10.07 eV - 10.36 eV = -0.29 eV,
B~
~
C6H5CCH+( ) + CS2 C6H5CCH + CS2+B
~
CS2+ would be observed if B of C6H5CCH+ is very long-lived.
exoergic!
70 72 74 76 78
0
50
100
++
+
..
.
C6H
4
C32
S34
S
C32
S2
III
III
IIII
I
Rel
ativ
e In
tens
ity
m/z
Partial mass spectrum obtained under the single focusing condition with C6H5
CCH and CS2 introduced into the ion source and collision cell, respectively. C6H
5CCH 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(Collision gases, their ionization energies(IIE) in eV , and success / failure tE) in eV , and success / failure to generate their ions by charge exchange with some precursor ionso generate their ions by charge exchange with some precursor ions
(CH3)2CHNH2 8.72 O O O O
1,3-C4H6 9.07 O X O (butadiene) CS2 10.07 O
CH3Br 10.54 O O O X X
C2H5Cl 10.98 X
CH3Cl 11.28 O X O X
C2H6 11.52 X O
O2 12.07 X
Xe 12.12 X X X CHF3 13.86 X
Precursor ionCollision gas IE, eV C6H5Cl+ • C6H5Br+ • C6H5CN+ • C6H5CCH+ • C6H5I+ • C6H5F+ •
Recombination energy (X) 9.066 8.991 9.71 8.75 8.754 9.20 Recombination energy (B) 11.330 10.633 11.84 10.36 9.771 13.81 *
~
~
Recombination energies of the X2B1, A2A2, and B2B2 states and the os
cillator strengths of the radiative transitions from the B2B2 states.
~ ~ ~~
State C6H5CCH+
X2B18.75
(0.0000000)
A2A2
9.34(0.0000004)
B2B2 10.36
Lowest quartet
C6H5Cl+
9.066(0.0000000)
9.707(0.0000008)
11.330
13.236
C6H5Br+
8.991(0.0000000)
9.663(0.0000001)
10.633
13.381
C6H5I+
8.754(0.0000000)
9.505(0.0000000)
9.771
12.664
C6H5CN+
9.71(0.0000000)
10.17(0.0000010)
11.84
13.3 12.7
Reaction threshold
~
~
~
12.356 11.891 11.07 12.725
Radiative decay of B2B2 is not efficient for all the cases.
B states are not dissociative.
The lowest quartet states lie ~2 eV above the B state. Relaxation by doublet – quartet intersystem crossing would not occur.
Internal conversion must be inefficient for the B states except for C6H5I+. For the B state of C6H5I+, internal conversion must be efficient.
~
~
~
~~
12.41
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
Vs
Ion source Magnetic analyzer Electrostatic analyzer
Vc
Collision cell
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
e- removal from a (C=C)
a (Xnp∥)
a (Xnp⊥)
AX~ 2
AA~ 2
BB~ 2
Hole states appearing in photoelectron spectra
Xnp
C2H4 C2H3X X
a
a
a
a ( Xnp ∥ character) a ( Xnp ⊥ character)
1) Vinyl chloride
Sharp vibrational bands for
Possibility of very long lifetime.
AA~ 2
2) Vinyl bromide
Sharp vibrational bands for
Possibility of very long lifetime.
AA~ 2
3) Vinyl iodide
Sharp vibrational bands for
Possibility of very long lifetime.
AA~ 2
X~
A~
C. CH2=CHCN, Acrylonitrile
Possibility of very long lifetime for AA~ 2
X~
A~
D. CH2=CHF, Vinyl fluoride
Broad bands
Short lifetime for
AA~ 2
AA~ 2
X~
A~
E. Experimental results
1) CH2=CHCl+
RE (CH2=CHCl+, ) = 11.664 eV
IE (CH3Cl) =11.28 eV
E = 11.28 eV – 11.664 eV = -0.384 eV,
A~
~
CH2=CHCl+( ) + CH3Cl CH2=CHCl + CH3Cl+ A~
CH3Cl+ would be observed if A of CH2=CHCl+ is very long-lived.
exoergic!
A state of CH2=CHCl+ is very long-lived.
20 30 40 50 60 70
(a)
IIII
I
Rel
ativ
e In
ten
sity
20 30 40 50 60 70
CH3
35Cl
+(b)
IIIIII
III
III
Rel
ativ
e In
ten
sity
m/z
Single – focusing mass spectrum recorded for C2H3Cl with CH3Cl introduced to the first cell.
Double – focusing mass spectrum
~
I I
2) CH2=CHBr+
RE (CH2=CHBr+, ) = 10.899 eV
IE (CH3Br) =10.54 eV
E = 10.54 eV – 10.899 eV = -0.359 eV,
A~
~
CH2=CHBr+( ) + CH3Br CH2=CHBr + CH3Br+ A~
CH3Br+ would be observed if A of CH2=CHBr+ is very long-lived.
exoergic!
90 92 94 96 98
CH3
81Br
+CH3
79Br
+
III
III
III
III
III
(b)
Rela
tive In
ten
sit
y
m/z
90 92 94 96 98
III
III
III
III
IIIII
(a)
Rela
tive In
ten
xit
y
A state of CH2=CHBr+ is very long-lived.~
3) CH2=CHI+
RE (CH2=CHI+, ) = 10.08 eV
IE (allene : CH2=C=CH2) =9.69 eV
E = 9.69 eV – 10.08 eV = -0.39 eV,
A~
~
CH2=CHI+( ) + CH2=C=CH2 CH2=CHI + CH2=C=CH2+ A
~
CH2=C=CH2 + would be observed if A of CH2=CHI+ is very long-lived.
exoergic!
35 37 39 41 43
C3H
4
+(b)
III
III
III
Rela
tive In
ten
sit
y
m/z
35 37 39 41 43
(a)
III
III
III
Rela
tive In
ten
sit
y
A state of CH2=CHI+ is very long-lived.~
4) CH2=CHCN+
RE (CH2=CHCN+, ) = 12.36 eV
IE (Xe) =12.12 eV
E = 12.12 eV – 12.36 eV = -0.24 eV,
A~
~
CH2=CHCN+( ) + Xe CH2=CHCN + Xe+ A~
Xe+ would be observed if A of CH2=CHCN+ is very long-lived.
exoergic!
124 126 128 130 132 134 136 138 140
(b)
IIIIII
III
III
III
III
III
Rela
tive In
ten
sit
y
m/z
124 126 128 130 132 134 136 138 140
(a)
IIIIII
III
III
III
III
III
Rela
tive In
ten
sit
y
A state of CH2=CHCN+ is very long-lived.~
5) CH2=CHF+
~
CH2=CHF+( ) + CH3F CH2=CHF + CH3F+ A~
CH3F+ would be observed if A of CH2=CHF+ is very long-lived.
exoergic!
RE (CH2=CHF+, ) = 13.80 eV
IE (CH3F) =12.50 eV
E = 12.50 eV – 13.80 eV = -1.3 eV,
A~
A state of CH2=CHF+ is not long-lived.~
20 30 40 50
(b)
(a)
I
I
I
I
Rela
tive In
ten
sit
y
20 30 40 50
Rela
tive In
ten
sit
y
m/z
Mass spectrum of C2H3F generated
by 20 eV EI recorded under the single focusing condition without CH3F.
Mass spectrum of C2H3F generated
by 20 eV EI recorded under the single focusing condition with CH3F.
Recombination energy (X) 10.005 9.804 9.35 10.91 10.63 Recombination energy (A) 11.664 10.899 10.08 12.36 13.80
Precursor ions Collision gas IE, eV C2H3Cl+ • C2H3Br+ • C2H3I+ • C2H3CN+ • C2H3F+•
~
~
X15.76 Ar
X12.50 CH3F
OX12.12 Xe
XXO11.28 CH3Cl
OXOO10.54 CH3Br
O 9.692 C3H4
(Allene)
OOOO 9.07 1,3-C4H6
(butadiene)
X
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.
C6H6+, CH2CHCl+,
C6H5Cl+, CH2CHBr+,
C6H5Br+, CH2CHI+,
C6H5CN+, CH2CHCN+,
C6H5CCH+,
Much more than found over the past 50 years!
AA~ 2
AA~ 2
AA~ 2
AA~ 2
22 BB
~2
2 BB~
22 BB
~2
2 BB~
g22EA
~