1. Chan Ho Kwon, Hong Lae Kim, and Myung Soo Kim* National Creative Research Initiative Center for Control of Reaction Dynamics and School of Chemistry, Seoul National University, Seoul 151-742, Korea Vibrational spectra of halobenzene cations in the ground and 2 B 2 electronic states obtained by one-photon mass-analyzed threshold ionization spectrometry Vibrational spectra of halobenzene cations in the ground and 2 B 2 electronic states obtained by one-photon mass-analyzed threshold ionization spectrometry

2. Contents Ⅰ. Motivation for research Ⅱ. Mass-analyzed threshold ionization (MATI) spectroscopy Ⅲ. MATI spectra in the ground electronic state Ⅳ. MATI spectra in the 2 B 2 excited electronic state Ⅴ. Selection rule Ⅵ. Summary and conclusion

3. 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’

4. B. Discovery of very long-lived excited electronic states of polyatomic ions states of polyatomic ions 1) Charge exchange ionization A + + B → A + B +  E = IE(B) - RE(A + ) IE: Ionization energy RE: 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 ≤ 0 Exoergicity criterion’

5. 2) Halobenzene and related ions Some electronic states of C 6 H 5 X + ( X = Cl, Br, I ) Ground state neutral Ground state neutral 3b 1, 1a 2 - e 1g of benzene 6b 2 - n(X3p ∥ ) 2b 1 - n(X3p ⊥ ) Ions Ions These are states appearing in photoelectron spectra. 3b13b1 1a21a2 6b26b2 2b12b1

6. C 6 H 5 C≡N + and C 6 H 5 C≡CH + Low – lying electronic states are similar to C 6 H 5 X + state - Loss of e - from  (C≡N ∥ ) or  (C≡C ∥ ) state - Loss of e - from  (C≡N ⊥ ) or  (C≡C ⊥ )

7. TABLE 1. Collision gases, their ionization energies(IE) in eV, and success / failure 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 12.24

8. Discovery  states of C 6 H 5 Cl +, C 6 H 5 Br +, C 6 H 5 CN +, C 6 H 5 CCH + are very long – lived ( > 10  s)  All the excited electronic states of C 6 H 5 F +, C 6 H 5 I + do not have long lifetimes.

9. Photoelectron spectra

10. Ⅱ. Mass-analyzed threshold ionization(MATI) spectroscopy A. Principle 1) Outline Photo-excite a molecule to a Rydberg state (high n) lying just below ( < 10cm -1 ) the ionization limit. Photo-excite a molecule to a Rydberg state (high n) lying just below ( < 10cm -1 ) the ionization limit. Some ions and electrons are generated by direct photoionization (direct ions/electrons). Remove these. Some ions and electrons are generated by direct photoionization (direct ions/electrons). Remove these. Ionize the molecule in Rydberg state (Rydberg neutral) by applying electric field (pulse-field ionization, PFI). Ionize the molecule in Rydberg state (Rydberg neutral) by applying electric field (pulse-field ionization, PFI). Scan h. Record spectrum by detecting Scan h. Record spectrum by detecting electrons → Zero electron kinetic energy spectrum (ZEKE). ions → MATI

11. 2) MATI 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.

12. 3) Lifetime of a Rydberg neutral Rydberg states (high n, low ℓ)  ∝ n 3 n = 200 → ~ 100 nsec ZEKE states (high n, ℓ, m )  ∝ n 4 n = 200 → ~ 20  sec A successful MATI detects ions from ZEKE states generated by PFI after a long delay time (  sec).

13. B. Photoexcitation h IE = 8 ~ 12eV (100 ~ 150nm) two-photon 1 + 1 one-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. h 1 h 2

14. C. Instrumentation 1) VUV laser Four-wave difference frequency mixing in Kr Four-wave difference frequency mixing in Kr h 1 h 2 h 3 h 4 4p64p6 5p[5/2] 2 5p[1/2] 0 h 1 = h 2 = 212.6 nm or 216.7 nm h 3 = 400 ~ 800 nm h 4 = 122 ~ 145 nm, 10 nJ

15. Four-wave sum frequency mixing in Hg Four-wave sum frequency mixing in Hg h 1 h 2 h 3 h 4 61S061S0 71S071S0 h 1 = h 2 = 312.8 nm h 3 = 340 ~ 650 nm h 4 = 107 ~ 126 nm, 20 nJ ~ 200nJ

16. 2) MATI spectrometer

17. (a) Top view dichroic mirror Kr cell MgF 2 lens photoionization chamber 50cm lens (b) Side view detector molecular beam VUV E3 E2 E1 G TOF

18. 3) Pulsing scheme E1 E2 E3 1200V 950V photoexcitation PFI delay

19. Ⅲ. MATI spectra in the ground electronic state Photon Energy, cm -1 C 6 H 5 35 Cl + C 6 H 5 37 Cl + Ion Signal

20. Photon Energy, cm -1 C 6 H 5 79 Br + C 6 H 5 81 Br +

21. Photon Energy, cm -1 C6H5I+C6H5I+C6H5I+C6H5I+ C6H5I+C6H5I+C6H5I+C6H5I+

22. C6H5F+C6H5F+C6H5F+C6H5F+ C6H5F+C6H5F+C6H5F+C6H5F+

23. Ionization energies (IE) to the ground ( 2 B 1 ) and 2 B 2 excited states of chloro-, bromo-, iodo-, and fluorobenzene cations, in eV Chlorobenzene9.0728 ± 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 ± 0.000610.6406 ± 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 2 B 1 2 B 2 IE( 2 B 1 ) IE( 2 B 2 ) Ref.

24. Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) chlorobenzene cation. Mode This work (Wilson) C 6 H 5 35 Cl + C 6 H 5 37 Cl + 972 600(?) 415 530 1114 1554 1592 1193 771 139 710 482 991 1408 829 1246 1661 2078 2225 950 1131 1360 1392 1527 1821 2277 974 600(?) 419 527 1118 1554 1593 1193 771 141 713 482 991 286 1411 838 1260 1677 2097 2235 950 1135 1368 1394 1533 1828 2280 975 422 531 1116 1200 716 394 995 311 1429 971 420 526 1115 1194 714 393 992 950 422 510 1100 1180 720 960 427 1121 1003 685 417 615 1093 1586 1598 1153 741 197 706 467 1026 287 1482 a1b1a1b2a1a1b2a1b1b1a1b1a1b2a1a1b1a1b2a1a1b2a1b1b1a1b1a1b2a1 1 4 6a 6b 7a 8a 8b 9a 10b 11 12 16b 18a 18b 19a 6a 2 6a 3 6a 4 6a 5 7a 2 6a 1 6b 1 6a 1 12 1 6a 2 6b 1 6a 1 1 1 6a 1 7a 1 7a 1 12 1 8a 1 12 1 symmetry Neutral PES MPI-PES MATI ZEKE

25. Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) bromobenzene cation. 3083(?) 331 593 1073 1577 1523 1193 791 126 678 1307 396 1008 257 1466 3083(?) 659 987 1322 1653 928 1402 1734 1754 1911 2061 2241 2474 3083(?) 329 593 1073 1577 1523 1193 791 126 678 1307 394 1008 257 1466 3083(?) 659 986 1320 1649 1399 1729 1750 1907 2058 2239 2471 Mode This work (Wilson) C 6 H 5 81 Br + 950 320 540 1100 1530 1180 720 980 331 1016 1001 3065 314 614 1070 1578 1176 1158 736 181 671 1321 409 1020 1472 3067 a1a1a1b2a1a1b2a1b2b1b1a1b2a2a1b2a1a1a1a1a1b2a1a1b2a1b2b1b1a1b2a2a1b2a1a1 1 2 6a 6b 7a 8a 8b 9a 9b 10b 11 12 14 16a 18a 18b 19a 20a 6a 2 6a 3 6a 4 6a 5 6a 1 6b 1 6a7a 6a 2 7a 7a12 6a8a 6a 3 7a 6a 2 8a 6a7a 2 symmetry Neutral PES MPI-PES C 6 H 5 79 Br +

26. Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) iodobenzene cation. Mode (Wilson) 990 284 538 1036 1575 1517 808 127 661 357 406 903 1015 242 567 848 1129 943 1226 1269 1296 1310 1548 1594 1648 1676 1695 2256 331 1016 998 268 612 1063 1575 729 167 654 398 421 903 1015 220 a1a1b2a1a1b2b1b1a1a2b1b1a1b2a1a1b2a1a1b2b1b1a1a2b1b1a1b2 1 6a 6b 7a 8a 8b 10b 11 12 16a 16b 17b 18a 18b 6a 2 6a 3 6a 4 6a 1 12 1 18b 1 1 1 6a 1 1 1 6a 1 18a 1 6a 1 7a 1 6a 2 1 1 6a 2 7a 1 1 1 12 1 12 1 18a 1 7a 1 12 1 6a 1 12 1 symmetry Neutral PES This work

27. Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) fluorobenzene cation. Mode (Wilson) 1299 500 606 1274 1610 1574 1168 1106 763 182 804 1339 1071 479 402 1502 1464 1668 1797 1842 2109 1968 2282 2343 500 505 1164 181 795 400 500 510 1620 1170 810 410 1301 517 615 1232 1604 1597 1156 1128 754 249 809 1326 1066 498 400 1500 1460 b2a1b2a1a1b2a1b2b1b1a1b2b2b1b2a1b2b2a1b2a1a1b2a1b2b1b1a1b2b2b1b2a1b2 3 6a 6b 7a 8a 8b 9a 9b 10b 11 12 14 15 16b 18b 19a 19b 6a 1 9a 1 6a 1 3 1 6a 1 14 1 6a 1 8a 1 9a 1 12 1 9a 1 9b 1 9a 2 symmetry Neutral MPI-PES MATI This work

28. 6a n progression Prominent for C 6 H 5 Cl +, C 6 H 5 Br +, C 6 H 5 I +. Not so for C 6 H 5 F +. Why? Calculation of geometrical change upon ionization. Calculation of mode eigenvectors for ions. ** B3LYP / 6-311++G * * and other levels.

29. geometry change upon ionization 6a eigenvector

30. Photon Energy, cm -1 cm -1 2 B 2, C 6 H 5 35 Cl + Ⅳ. MATI spectra in the B 2 B 2 excited electronic state ~

31. Photon Energy, cm -1 cm -1 2 B 2, C 6 H 5 79 Br +

32. Vibrational frequencies (in cm -1 ) and their assignments for the chlorobenzene cation in the 2 B 2 excited state. Mode (Wilson) 961 1279 667 382 546 1080 1173 759 153 725 329 439 899 1009 246 709 870 1010 730 384 562 1131 1263 636 223 218 866 329 970 340 1003 1271 682 985 420 616 1085 1174 830 740 196 701 400 467 902 1026 297 a1b2b1b1a1b2a1a1a2b1b1a1a2b1b1a1b2a1b2b1b1a1b2a1a1a2b1b1a1a2b1b1a1b2 1 3 4 5 6a 6b 7a 9a 10a 10b 11 12 16a 16b 17b 18a 18b 6a 1 16a 1 6b 1 16a 1 symmetry Neutral PES REMPDS PIRI This work 869 387 943 761 313 260

33. Mode (Wilson) 959 1251 542 1015 1571 1180 1130 622 1333 889 982 1419 970 620 1001 1264 614 1070 1578 1176 1158 671 1321 904 1020 1472 a1b2b2a1a1a1b2a1b2b1a1a1a1b2b2a1a1a1b2a1b2b1a1a1 1 3 6b 7a 8a 9a 9b 12 14 17b 18a 19a symmetry Neutral PES This work Vibrational frequencies (in cm -1 ) and their assignments for the bromobenzene cation in the 2 B 2 excited state.

34. Photon Energy, cm -1 2 B 2, C 6 H 5 I +

35. Ⅴ. Selection rule Theoretical Transition moment for the R (Rydberg) ← X (ground) transition Born - Oppenheimer approximation Ground state→ zero–point level ( ∵ beam condition), totally symmetry (a 1 ) → vibrational state of R should be a 1 also. → vibrational state of R should be a 1 also. a 1 propensity rule observation a 1 > b 2 > b 1 >> a 2 Why? ~ ~  RX =  RX =

36. Ⅵ. Summary and conclusion 1.MATI spectra of C 6 H 5 X + in the ground ( X = Cl, Br, I, F ) and B 2 B 2 excited ( X = Cl, Br, I ) electronic states obtained by one–photon VUV- MATI spectroscopy. 2.Accurate ionization energies and vibrational frequencies in the ground ( X = Cl, Br, I, F ) and B 2 B 2 excited ( X = Cl, Br ) electronic states determined. 3.The ground state MATI spectra ( X = Cl, Br, I ) display prominent 6a n progression due to geometry change upon ionization along the 6a eigenvector. 4.Well-resolved vibrational spectra obtained for B 2 B 2 of C 6 H 5 Cl + and C 6 H 5 Br + which are very long-lived states. Broad band spectrum obtained for B 2 B 2 of C 6 H 5 I + which has a short lifetime. ~ ~ ~ ~ 5. A routine spectroscopic technique, VUV-MATI, has been developed to record vibrational spectra of polyatomic ions.