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Vibrational spectra of halobenzene cations in the ground ~ 2B electronic states obtained by one-photon and B 2 mass-analyzed threshold ionization spectrometry 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 Contents Ⅰ. Motivation for research Ⅱ. Mass-analyzed threshold ionization (MATI) spectroscopy Ⅲ. MATI spectra in the ground electronic state ~ 2B excited electronic state Ⅳ. MATI spectra in the B 2 Ⅴ. Selection rule Ⅵ. Summary and conclusion 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’ B. Discovery of very long-lived excited electronic 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’ 2) Halobenzene and related ions Some electronic states of C6H5X+• ( X = Cl, Br, I ) Ground state neutral 3b1 3b1 , 1a2 - e1g of benzene 1a2 6b2 6b2 - n(X3p∥) 2b1 - n(X3p⊥) 2b1 Ions ~ (b1)-1 X2 B1 ~ (a2 )-1 A2 A2 ~ (b2 )-1B2 B2 ~ (b1)-1C2 B1 These are states appearing in photoelectron spectra. C6H5C≡N+• and C6H5C≡CH+• Low – lying electronic states are similar to C6H5X+• ~ B state - Loss of e- from (C≡N∥) or (C≡C∥) ~ C state - Loss of e- from (C≡N⊥) or (C≡C⊥) TABLE 1. Collision gases, their ionization energies(IE) in eV, and success / failure to generate their ions by charge exchange with some precursor ions Collision gas Precursor ions IE, eV C6H5Cl+ (CH3)2CHNH2 1,3-C4H6 (butadiene) CS2 8.72 O 10.07 CH3Br 10.54 C2H5Cl 10.98 CH3Cl 11.28 O C2H6 11.52 X O2 12.07 Xe 12.12 CHF3 13.86 C6H5Br+ C6H5CN+ C6H5CCH+ O 9.07 ~) Recombination energy( X O O C6H5I+ C6H5F+ O X O O O O O X X X X O X O X X X X X 9.066 8.991 9.71 8.75 8.754 9.20 ~ ) 11.330 Recombination energy( B 10.633 11.84 10.36 9.771 12.24 Discovery ~ states of C H Cl+• , C H Br+• , C H CN+• , C H CCH+• B 6 5 6 5 6 5 6 5 are very long – lived ( > 10 s) All the excited electronic states of C6H5F+• , C6H5I+• do not have long lifetimes. Photoelectron spectra 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. 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). Scan h. Record spectrum by detecting electrons → Zero electron kinetic energy spectrum (ZEKE). ions → MATI 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. 3) Lifetime of a Rydberg neutral Rydberg states (high n , low ℓ) ∝ n3 n = 200 → ~ 100 nsec ZEKE states (high n , ℓ , m ) ∝ n4 n = 200 → ~ 20 sec A successful MATI detects ions from ZEKE states generated by PFI after a long delay time (sec). B. Photoexcitation h2 h IE = 8 ~ 12eV (100 ~ 150nm) h1 two-photon 1 + 1 one-photon Two-photon MATI Difficult to control multiphoton processes. Applicable to systems with a stable intermediate state with E < 5.6 eV = 220nm. For most neutrals, 1st excited states are not stable. One-photon MATI No complications as above. Requires vacuum ultraviolet (VUV) laser. C. Instrumentation 1) VUV laser Four-wave difference frequency mixing in Kr 5p[1/2]0 h3 5p[5/2]2 h2 h1 = h2 = 212.6 nm or 216.7 nm h4 h1 4p6 h3 = 400 ~ 800 nm h4 = 122 ~ 145 nm, 10 nJ Four-wave sum frequency mixing in Hg h3 71S0 h1 = h2 = 312.8 nm h2 h4 h3 = 340 ~ 650 nm h4 = 107 ~ 126 nm, 20 nJ ~ 200nJ h1 61S0 2) MATI spectrometer TOF MCP Ar Out Out Temperature-controlled pulsed valve LiF lens UV, S Achromatic lens Hg Water in Water in Heating block (b) Side view (a) Top view detector photoionization chamber MgF2 lens Kr cell dichroic mirror 50cm lens TOF molecular beam G E1 E2 E3 VUV 3) Pulsing scheme photoexcitation E1 950V E2 1200V E3 PFI delay Ⅲ. MATI spectra in the ground electronic state Ion Signal C6H535Cl+• C6H537Cl+• Photon Energy, cm-1 Ion Signal C6H579Br+• C6H581Br+• Photon Energy, cm-1 C6H5I+• Photon Energy, cm-1 C6H5F+• Photon Energy, cm-1 ~ 2B ) and ~ 2B excited states Ionization energies (IE) to the ground (X B 2 1 of chloro-, bromo-, iodo-, and fluorobenzene cations, in eV ~2B ) IE( X 1 ~2B ) IE( B 2 Ref. 9.0728 ± 0.0006 9.0723 ± 0.0006 9.0720 ± 0.0006 9.066 ± 0.008 11.3327 ± 0.0006 Bromobenzene 8.9976 ± 0.0006 8.991 ± 0.008 8.98 ± 0.02 10.6406 ± 0.0006 10.633 ± 0.008 Iodobenzene 8.7580 ± 0.0006 8.754 ± 0.008 8.77 ± 0.02 This work PES PEPICO Fluorobenzene 9.2033 ± 0.0006 9.2033 ± 0.0006 9.2044 ± 0.0005 9.18 ± 0.02 This work MATI ZEKE MPI-PES Chlorobenzene 11.330 ± 0.008 This work MATI ZEKE PES This work PES MPI-PES Vibrational frequencies (in cm-1) and their assignments for the ~2B ) chlorobenzene cation. ground state ( X 1 Mode symmetry (Wilson) 1 4 6a 6b 7a 8a 8b 9a 10b 11 12 16b 18a 18b 19a 6a2 6a3 6a4 6a5 7a2 6a16b1 6a1121 6a26b1 6a111 6a17a1 7a1121 8a1121 a1 b1 a1 b2 a1 a1 b2 a1 b1 b1 a1 b1 a1 b2 a1 Neutral 1003 685 417 615 1093 1586 1598 1153 741 197 706 467 1026 287 1482 PES 427 1121 MPI-PES MATI ZEKE 950 971 975 422 510 1100 420 526 1115 422 531 1116 1180 1194 1200 720 714 393 992 716 394 995 311 1429 960 This work C6H535Cl+• C6H5 37Cl+• 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 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 Vibrational frequencies (in cm-1) and their assignments for the ~ 2B ) bromobenzene cation. ground state ( X 1 Mode symmetry (Wilson) 1 2 6a 6b 7a 8a 8b 9a 9b 10b 11 12 14 16a 18a 18b 19a 20a 6a2 6a3 6a4 6a5 6a16b1 6a7a 6a27a 7a12 6a8a 6a37a 6a28a 6a7a2 a1 a1 a1 b2 a1 a1 b2 a1 b2 b1 b1 a1 b2 a2 a1 b2 a1 a1 Neutral 1001 3065 314 614 1070 1578 1176 1158 736 181 671 1321 409 1020 1472 3067 PES MPI-PES This work C6H579Br+• C6H581Br+• 950 331 320 540 1100 1530 3083(?) 331 593 1073 1577 1523 1193 3083(?) 329 593 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 791 126 678 1307 394 1008 257 1466 3083(?) 659 986 1320 1649 1180 720 1016 980 1399 1729 1750 1907 2058 2239 2471 Vibrational frequencies (in cm-1) and their assignments for ~ the ground state ( X2B1) iodobenzene cation. Mode (Wilson) 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 a1 a1 b2 a1 a1 b2 b1 b1 a1 a2 b1 b1 a1 b2 998 268 612 1063 1575 729 167 654 398 421 903 1015 220 PES 331 1016 This work 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 Vibrational frequencies (in cm-1) and their assignments for ~ the ground state ( X2B1) fluorobenzene cation. Mode (Wilson) 3 6a 6b 7a 8a 8b 9a 9b 10b 11 12 14 15 16b 18b 19a 19b 6a19a1 6a131 6a1141 6a18a1 9a1121 9a19b1 9a2 symmetry Neutral b2 a1 b2 a1 a1 b2 a1 b2 b1 b1 a1 b2 b2 b1 b2 a1 b2 1301 517 615 1232 1604 1597 1156 1128 754 249 809 1326 1066 498 400 1500 1460 MPI-PES 500 510 MATI 500 505 1620 1170 1164 810 181 795 410 400 This work 1299 500 606 1274 1610 1574 1168 1106 763 182 804 1339 1071 479 402 1502 1464 1668 1797 1842 2109 1968 2282 2343 6an 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 ~ Ⅳ. MATI spectra in the B2B2 excited electronic state ~ B 2B2 , C6H535Cl+• cm-1 Photon Energy, cm-1 ~ B 2B2, C6H579Br+• cm-1 Photon Energy, cm-1 Vibrational frequencies (in cm-1) and their assignments for ~ 2B excited state. the chlorobenzene cation in the B 2 Mode (Wilson) symmetry Neutral 1 3 4 5 6a 6b 7a 9a 10a 10b 11 12 16a 16b 17b 18a 18b 6a116a1 6b116a1 a1 b2 b1 b1 a1 b2 a1 a1 a2 b1 b1 a1 a2 b1 b1 a1 b2 1003 1271 682 985 420 616 1085 1174 830 740 196 701 400 467 902 1026 297 PES REMPDS PIRI This work 970 869 1010 961 1279 667 340 387 943 730 384 562 1131 1263 382 546 1080 1173 761 313 260 636 223 218 866 329 759 153 725 329 439 899 1009 246 709 870 Vibrational frequencies (in cm-1) and their assignments for ~ 2B excited state. the bromobenzene cation in the B 2 Mode (Wilson) 1 3 6b 7a 8a 9a 9b 12 14 17b 18a 19a symmetry Neutral PES This work a1 b2 b2 a1 a1 a1 b2 a1 b2 b1 a1 a1 1001 1264 614 1070 1578 1176 1158 671 1321 904 1020 1472 970 959 1251 542 1015 1571 1180 1130 622 1333 889 982 1419 620 ~ B 2B2, C6H5 I+• Photon Energy, cm-1 Ⅴ. Selection rule Theoretical Transition moment for the R (Rydberg) ← X (ground) transition RX = < R││ X > ~ < elR││elX > < vibR│vibX > Born - Oppenheimer approximation Ground state → zero–point level (∵ beam condition), totally symmetry (a1) → vibrational state of R should be a1 also. a1 propensity rule observation a1 > b2 > b1 >> a2 Why? Ⅵ. Summary and conclusion ~ 1. MATI spectra of C6H5X+• in the ground ( X = Cl, Br, I, F ) and B2B2 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 B2B2 excited ( X = Cl, Br ) electronic states determined. 3. The ground state MATI spectra ( X = Cl, Br, I ) display prominent 6an progression due to geometry change upon ionization along the 6a eigenvector. ~ 4. Well-resolved vibrational spectra obtained for B2B2 of C6H5Cl+• and C6H5Br+• which are very long-lived states. Broad band ~ spectrum obtained for B2B2 of C6H5I+• which has a short lifetime. 5. A routine spectroscopic technique, VUV-MATI, has been developed to record vibrational spectra of polyatomic ions.