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Ⅰ. Motivation for research A. Excited electronic states of polyatomic ions Cases of very long-lived (‘metastable’) excited electronic states are very rare for polyatomic (n≥4) ions. Decay mechanisms ( ⅰ ) Internal conversion to the ground electronic state ( ⅱ ) Dissociation on a repulsive electronic state ( ⅲ ) Radiative decay Absolute prevalence of ( ⅰ ) has led to the theory of mass spectra (RRKM-QET) ‘Molecular ions undergo internal conversion to the ground state and dissociate statistically (RRKM or microcanonical transition state theory) there in’
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Chan Ho Kwon, Hong Lae Kim, and Myung Soo Kim*Chan Ho Kwon, Hong Lae Kim, and Myung Soo Kim*
National Creative Research Initiative Center for Control of Reaction National Creative Research Initiative Center for Control of Reaction Dynamics and School of Chemistry, Seoul National University, Dynamics and School of Chemistry, Seoul National University, Seoul 151-742, KoreaSeoul 151-742, Korea
Vibrational spectra of halobenzene cations in the ground Vibrational spectra of halobenzene cations in the ground
and and 22BB22 electronic states obtained by one-photon electronic states obtained by one-photon
mass-analyzed threshold ionization spectrometrymass-analyzed threshold ionization spectrometry
B~
Contents
ⅠⅠ. . Motivation for researchMotivation for research
ⅡⅡ . Mass-analyzed threshold ionization (MAT. Mass-analyzed threshold ionization (MATII) spectroscopy) spectroscopy
ⅢⅢ. . MATMATII spectra in the ground electronic state spectra in the ground electronic state
ⅣⅣ. . MATMATII spectra in the spectra in the 22BB22 excited electronic state excited electronic state
ⅤⅤ. . Selection ruleSelection rule
ⅥⅥ. . Summary and conclusionSummary and conclusion
B~
ⅠⅠ. . Motivation for researchMotivation for research
A. Excited electronic states of polyatomic ionsA. Excited electronic states of polyatomic ionsCases of very long-lived (‘metastable’) excited electronic states are very rare for polyatomic (n≥4) ions.
Decay mechanisms
( ) ⅰ Internal conversion to the ground electronic state( ) ⅱ Dissociation on a repulsive electronic state( ) ⅲ Radiative decay
Absolute prevalence of Absolute prevalence of ( )ⅰ( )ⅰ has led to the theory of mass has led to the theory of mass spectra (RRKM-QET)spectra (RRKM-QET)
‘Molecular ions undergo internal conversion to the ground state and dissociate statistically (RRKM or microcanonical transition state theory) there in’
B. Discovery of very long-lived excited electronicB. Discovery of very long-lived excited electronic states of polyatomic ionsstates of polyatomic ions
1) 1) Charge exchange ionizationCharge exchange ionization
A+ + B → A + B+
E = IE(B) - RE(A+)
IE : Ionization energyRE: Recombination energy of A+
= Ionization energy of A to the state in which A+ is in.
For charge exchange under near thermal condition involving polyatomics, cross section is very large only when
E ≤ 0Exoergicity criterion’
2) 2) Halobenzene and related ions Halobenzene and related ions
Some electronic states of C6H5X+• ( X = Cl, Br, I )
Ground state neutralGround state neutral
3b1 , 1a2 - e1g of benzene
6b2 - n(X3p∥)
2b1 - n(X3p⊥)
IIons ons
These are states appearing in photoelectron spectra.
3b1
1a2
6b2
2b1
121
1 BX~)b( -2
212 AA~)a( -
221
2 BB~)b( -1
211 BC~)b( -
C6H5C≡N+• and C6H5C≡CH+•
Low – lying electronic states are similar to C6H5X+•
state - Loss of e- from (C≡N∥) or (C≡C∥)B~
state - Loss of e- from (C≡N⊥) or (C≡C⊥)C~
TABLE 1TABLE 1. Collision gases, their ionization energies(. Collision gases, their ionization energies(IIE) in eV, and success / failure E) in eV, and success / failure to generate their ions by charge exchange with some precursor ions to generate their ions by charge exchange with some precursor ions
Recombination energy( ) 9.066 8.991 9.71 8.75 8.754 9.20
Recombination energy( ) 11.330 10.633 11.84 10.36 9.771 12.24B~X~
Discovery
states of C6H5Cl+• , C6H5Br+• , C6H5CN+• , C6H5CCH+•
are very long – lived ( > 10 s)
All the excited electronic states of C6H5F+• , C6H5I+•
do not have long lifetimes.
B~
Photoelectron spectraPhotoelectron spectra
ⅡⅡ. . Mass-analyzed threshold ionization(MATMass-analyzed threshold ionization(MATII) spectroscopy) spectroscopy
A. PrincipleA. Principle1) 1) OutlineOutline
Photo-excite a molecule to a Rydberg state (high n) lying just Photo-excite a molecule to a Rydberg state (high n) lying just below ( < 10cmbelow ( < 10cm-1-1) the ionization limit.) the ionization limit.
Some ions and electrons are generated by direct Some ions and electrons are generated by direct photoionization (direct ions/electrons). Remove these.photoionization (direct ions/electrons). Remove these.
IIonize the molecule in Rydberg state (Rydberg neutral) byonize the molecule in Rydberg state (Rydberg neutral) byapplying electric field (pulse-field ionization, PFapplying electric field (pulse-field ionization, PFII).).
Scan hScan h. Record spectrum by detecting. Record spectrum by detectingelectrons → Zero electron kinetic energy spectrum (ZEKE). electrons → Zero electron kinetic energy spectrum (ZEKE).
ions → MATions → MATII
2) 2) MATMATII vs. ZEKE vs. ZEKE
Weakness
Poor resolution [ZEKE : 5cm-1 (conventional), 0.1 cm-1 (high resolution), MATI : 10cm-1], related to removal of heavy ions compared to removal of e- in ZEKE.
Strength
Identification of ions contributing to each peak.
Generation of state-selected ions.
3) 3) Lifetime of a Rydberg neutralLifetime of a Rydberg neutral
Rydberg states (high n , low ℓ)
∝ n3
n = 200 → ~ 100 nsecn = 200 → ~ 100 nsec
ZEKE states (high n , ℓ , m )
∝ n4
n = 200 → ~ 20 n = 200 → ~ 20 secsec
A successful MATI detects ions from ZEKE states generated by PFI after a long delay time (sec).
B. PhotoexcitationB. Photoexcitation
h IE = 8 ~ 12eV (100 ~ 150nm)
two-photon 1 + 1two-photon 1 + 1 one-photonone-photon
Two-photon MATI
Difficult to control multiphoton processes.Difficult to control multiphoton processes.
Applicable to systems with a stable intermediate state with E < 5.6 eV Applicable to systems with a stable intermediate state with E < 5.6 eV = 220nm. For most neutrals, 1st excited states are not stable.= 220nm. For most neutrals, 1st excited states are not stable.
One-photon MATI
No complications as above.No complications as above.
Requires vacuum ultraviolet (VUV) laser.Requires vacuum ultraviolet (VUV) laser.
h1
h2
C. C. IInstrumentationnstrumentation
1) 1) VUV laserVUV laser
Four-wave difference frequency mixing in KrFour-wave difference frequency mixing in Kr
h1
h2
h3
h4
4p6
5p[5/2]2
5p[1/2]0
h1 = h2 = 212.6 nm or 216.7 nm
h3 = 400 ~ 800 nm
h4 = 122 ~ 145 nm, 10 nJ
Four-wave sum frequency mixing in HgFour-wave sum frequency mixing in Hg
h1
h2
h3
h4
61S0
71S0
h1 = h2 = 312.8 nm
h3 = 340 ~ 650 nm
h4 = 107 ~ 126 nm, 20 nJ ~ 200nJ
TO
F
MCP
LiF lens
Achromaticlens
UV, S
Temperature-controlledpulsed valve
Hg
Heatingblock
Ar
Water inWater in
Out Out
2) 2) MATMATII spectrometer spectrometer
((a) Top viewa) Top view
dichroic mirrorKr cellMgF2 lens
photoionization chamber
50cm lens
((b) Side viewb) Side view
detector
molecular beam VUVE3E2E1G
TOF
3) 3) Pulsing schemePulsing scheme
E1
E2
E31200V
950V
photoexcitation
PFI delay
ⅢⅢ. . MATMATII spectra in the ground spectra in the ground electronic stateelectronic state
Photon Energy, cm-1
CC66HH553535ClCl+•+•
CC66HH553737ClCl+•+•Io
n Si
gnal
Ion
Sign
al
Photon Energy, cm-1
CC66HH557979BrBr+•+•
CC66HH558181BrBr+•+•
Photon Energy, cm-1
CC66HH55II+•+•
Photon Energy, cm-1
CC66HH55FF+•+•
IIonization energies (onization energies (IIE) to the ground ( E) to the ground ( 22BB11) and ) and 22BB22 excited states excited states of chloro-, bromo-, iodo-, and fluorobenzene cations, in eVof chloro-, bromo-, iodo-, and fluorobenzene cations, in eV
Chlorobenzene 9.0728 ± 0.0006 11.3327 ± 0.0006 This work 9.0723 ± 0.0006 MATI
9.0720 ± 0.0006 ZEKE 9.066 ± 0.008 11.330 ± 0.008 PES
Bromobenzene 8.9976 ± 0.0006 10.6406 ± 0.0006 This work 8.991 ± 0.008 10.633 ± 0.008 PES
8.98 ± 0.02 MPI-PES
Iodobenzene 8.7580 ± 0.0006 This work 8.754 ± 0.008 PES 8.77 ± 0.02 PEPICO
Fluorobenzene 9.2033 ± 0.0006 This work 9.2033 ± 0.0006 MATI
9.2044 ± 0.0005 ZEKE 9.18 ± 0.02 MPI-PES
IE( 22BB11) IE( 22BB22) Ref.B~X~
X~ B~
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for the and their assignments for the ground state ( ground state ( 22BB11) chlorobenzene cation.) chlorobenzene cation.
Mode This work(Wilson) C6H5
35Cl+• C6H5 37Cl+•
972600(?)
415530
1114155415921193
771139710482991
1408829
1246166120782225
950113113601392152718212277
974600(?)
419527
1118155415931193 771141713482991286
1411838
1260167720972235
950113513681394 1533 1828 2280
975
422531
1116
1200
716394995311
1429
971
420526
1115
1194
714393992
950
422510
1100
1180
720
960
427
1121
1003685417615
1093158615981153
741 197
706467
1026287
1482
a1b1a1b2a1a1b2a1b1b1a1b1a1b2a1
1 46a6b 7a 8a 8b9a 10b1112 16b 18a18b19a6a2
6a3
6a4
6a5 7a2
6a16b1
6a1121
6a26b1 6a111 6a17a1
7a1121 8a1121
symmetry Neutral PES MPI-PES MATI ZEKE
X~
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for the and their assignments for the ground state ( ground state ( 22BB11) bromobenzene cation.) bromobenzene cation.X~
3083(?)331593
1073157715231193
791126678
1307396
1008257
14663083(?)
659987
13221653928
1402173417541911206122412474
3083(?) 329 593 1073 1577 1523 1193
791 126 678 1307 394 1008 257 14663083(?) 659 986 1320 1649
1399 1729 1750 1907 2058 2239
2471
Mode This work(Wilson) C6H5
81Br+•
950
320540
11001530
1180
720
980
331
1016
10013065
314614
10701578
11761158
736181671
1321409
1020
14723067
a1a1 a1b2a1a1b2a1b2b1b1a1b2a2a1b2a1a1
1 26a6b 7a 8a 8b9a 9b10b1112 1416a 18a18b19a20a6a2
6a3
6a4
6a5 6a16b1 6a7a6a27a 7a12 6a8a6a37a 6a28a 6a7a2
symmetry Neutral PES MPI-PESC6H5
79Br+•
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the ground state ( the ground state ( 22BB11) iodobenzene cation.) iodobenzene cation.
Mode (Wilson)
990 284 538103615751517 808 127 661 357 406 9031015 242 567 8481129 9431226126912961310154815941648167616952256
331
1016
998 268 61210631575
729 167 654 398 421 9031015 220
a1a1 b2a1a1b2b1b1a1a2 b1 b1a1b2
1 6a 6b 7a 8a 8b 10b 11 12 16a 16b 17b 18a 18b 6a2
6a3
6a4 6a1121 18b111 6a111
6a118a1
6a17a1 6a211 6a27a1 11121
12118a1 7a1121
6a1121
symmetry Neutral PES This work
X~
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the ground state ( the ground state ( 22BB11) fluorobenzene cation.) fluorobenzene cation.
Mode(Wilson)
1299 500 60612741610157411681106 763 182 80413391071 479 402150214641668179718422109196822822343
500 505
1164
181 795
400
500510
1620
1170
810
410
1301 517 61512321604159711561128 754 249 80913261066 498 40015001460
b2 a1b2 a1 a1b2a1b2b1b1a1b2b2b1b2a1b2
36a6b 7a 8a 8b9a 9b10b1112 141516b 18b19a19b 6a19a1 6a131
6a1141 6a18a1 9a1121 9a19b1 9a2
symmetry Neutral MPI-PES MATI This work
X~
66aann progression progression
Prominent for C6H5Cl+• , C6H5Br+• , C6H5I+•.
Not so for C6H5F+• . Why?
Calculation of geometrical change upon ionization.
Calculation of mode eigenvectors for ions.
B3LYP / 6-311++G ** ** and other levels.
geometry change upon ionization 6a eigenvector
Photon Energy, cm-1
cmcm-1-1
2B2 , C6H535Cl+•B~
ⅣⅣ. . MATMATII spectra in the Bspectra in the B22BB22 excited electronic state excited electronic state~
Photon Energy, cm-1
cm-1
2B2, C6H579Br+•B~
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the chlorobenzene cation in the the chlorobenzene cation in the 22BB2 2 excited state. excited state.
Mode(Wilson)
9611279 667
382 54610801173
759 153 725 329 439 8991009 246 709 870
1010
730 384 56211311263
636 223 218
866 329
970
340
1003 1271 682 985 420 61610851174 830 740 196 701 400 467 9021026 297
a1
b2 b1 b1
a1
b2
a1
a1
a2
b1
b1
a1
a2
b1
b1 a1
b2
13456a6b 7a 9a 10a10b1112 16a16b17b 18a18b 6a116a1 6b116a1
symmetry Neutral PES REMPDS PIRI This work
869
387
943
761
313
260
B~
Mode(Wilson)
9591251 5421015157111801130 6221333 889 9821419
970
620
10011264 6141070157811761158 6711321 90410201472
a1
b2 b2 a1
a1
a1
b2
a1
b2
b1
a1
a1
136b 7a 8a9a 9b12 1417b 18a19a
symmetry Neutral PES This work
Vibrational frequencies (in cmVibrational frequencies (in cm-1-1) ) and their assignments for and their assignments for the bromobenzene cation in the the bromobenzene cation in the 22BB2 2 excited state. excited state. B~
Photon Energy, cm-1
2B2, C6H5 I+•B~
ⅤⅤ. . Selection ruleSelection rule
Theoretical
Transition moment for the R (Rydberg) ← X (ground) transitionTransition moment for the R (Rydberg) ← X (ground) transition
Born - Oppenheimer Born - Oppenheimer approximationapproximation
Ground stateGround state → zero–point level ( beam condition),∵→ zero–point level ( beam condition),∵totally symmetry (atotally symmetry (a11))
→ → vibrational state of R should be avibrational state of R should be a11 also. also.aa11 propensity rulepropensity rule
observation
aa11 > b > b22 > b > b11 >> a >> a22 Why? Why?
~ < ~ < elelRR││││elel
XX > < > < vibvibRR││vibvib
XX > >
RXRX = < = < RR││││ XX > >
ⅥⅥ. . Summary and conclusionSummary and conclusion
1.1. MATMATII spectra of C spectra of C66HH55XX+• +• in the ground ( X = Cl, Br, in the ground ( X = Cl, Br, II, F ) and B, F ) and B22BB22 excited ( X = Cl, Br, excited ( X = Cl, Br, I I ) electronic states obtained by one–photon ) electronic states obtained by one–photon VUV- MATVUV- MATII spectroscopy. spectroscopy.
2.2. Accurate ionization energies and vibrational frequencies in the Accurate ionization energies and vibrational frequencies in the ground ( X = Cl, Br, ground ( X = Cl, Br, II, F ) and B, F ) and B22BB22 excited ( X = Cl, Br ) electronic excited ( X = Cl, Br ) electronic states determined.states determined.
3.3. The ground state MATThe ground state MATII spectra ( X = Cl, Br, spectra ( X = Cl, Br, II ) display prominent ) display prominent 6a6ann progression due to geometry change upon ionization along progression due to geometry change upon ionization along the 6a eigenvector.the 6a eigenvector.
4.4. Well-resolved vibrational spectra obtained for BWell-resolved vibrational spectra obtained for B22BB22 of C of C66HH55ClCl+•+• and and CC66HH55BrBr+•+• which are very long-lived states. Broad band spectrum which are very long-lived states. Broad band spectrum obtained for Bobtained for B22BB22 of C of C66HH55II+•+• which has a short lifetime. which has a short lifetime.
~
~
~
~
5. A routine spectroscopic technique, VUV-MATI, has been developed to record vibrational spectra of polyatomic ions.