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 , 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

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

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 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 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

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 ⊥ )

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( ) Recombination energy( )

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.

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. 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

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)  ∝ 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).

B. Photoexcitation h IE = 8 ~ 12eV (100 ~ 150nm) two-photon 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

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 = nm or nm h 3 = 400 ~ 800 nm h 4 = 122 ~ 145 nm, 10 nJ

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 = nm h 3 = 340 ~ 650 nm h 4 = 107 ~ 126 nm, 20 nJ ~ 200nJ

2) MATI spectrometer

(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

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

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

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

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

C6H5F+C6H5F+C6H5F+C6H5F+ C6H5F+C6H5F+C6H5F+C6H5F+

Ionization energies (IE) to the ground ( 2 B 1 ) and 2 B 2 excited states of chloro-, bromo-, iodo-, and fluorobenzene cations, in eV Chlorobenzene ± ± This work ± MATI ± ZEKE ± ± PES Bromobenzene ± ± This work ± ± PES 8.98 ± 0.02 MPI-PES Iodobenzene ± This work ± PES 8.77 ± 0.02 PEPICO Fluorobenzene ± This work ± MATI ± ZEKE 9.18 ± 0.02 MPI-PES 2 B 1 2 B 2 IE( 2 B 1 ) IE( 2 B 2 ) Ref.

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 (?) (?) a1b1a1b2a1a1b2a1b1b1a1b1a1b2a1a1b1a1b2a1a1b2a1b1b1a1b1a1b2a a 6b 7a 8a 8b 9a 10b b 18a 18b 19a 6a 2 6a 3 6a 4 6a 5 7a 2 6a 1 6b 1 6a a 2 6b 1 6a a 1 7a 1 7a a symmetry Neutral PES MPI-PES MATI ZEKE

Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) bromobenzene cation. 3083(?) (?) (?) (?) Mode This work (Wilson) C 6 H 5 81 Br a1a1a1b2a1a1b2a1b2b1b1a1b2a2a1b2a1a1a1a1a1b2a1a1b2a1b2b1b1a1b2a2a1b2a1a a 6b 7a 8a 8b 9a 9b 10b a 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 +

Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) iodobenzene cation. Mode (Wilson) a1a1b2a1a1b2b1b1a1a2b1b1a1b2a1a1b2a1a1b2b1b1a1a2b1b1a1b2 1 6a 6b 7a 8a 8b 10b a 16b 17b 18a 18b 6a 2 6a 3 6a 4 6a b a a 1 18a 1 6a 1 7a 1 6a a 2 7a a 1 7a a symmetry Neutral PES This work

Vibrational frequencies (in cm -1 ) and their assignments for the ground state ( 2 B 1 ) fluorobenzene cation. Mode (Wilson) b2a1b2a1a1b2a1b2b1b1a1b2b2b1b2a1b2b2a1b2a1a1b2a1b2b1b1a1b2b2b1b2a1b2 3 6a 6b 7a 8a 8b 9a 9b 10b b 18b 19a 19b 6a 1 9a 1 6a a a 1 8a 1 9a a 1 9b 1 9a 2 symmetry Neutral MPI-PES MATI This work

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 / G * * and other levels.

geometry change upon ionization 6a eigenvector

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 ~

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

Vibrational frequencies (in cm -1 ) and their assignments for the chlorobenzene cation in the 2 B 2 excited state. Mode (Wilson) a1b2b1b1a1b2a1a1a2b1b1a1a2b1b1a1b2a1b2b1b1a1b2a1a1a2b1b1a1a2b1b1a1b a 6b 7a 9a 10a 10b a 16b 17b 18a 18b 6a 1 16a 1 6b 1 16a 1 symmetry Neutral PES REMPDS PIRI This work

Mode (Wilson) a1b2b2a1a1a1b2a1b2b1a1a1a1b2b2a1a1a1b2a1b2b1a1a b 7a 8a 9a 9b b 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.

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

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 =

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.