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Probing very long-lived excited electronic states of molecular cations by mass spectrometry

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.

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Probing very long-lived excited electronic states of molecular cations by mass spectrometry

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  1. 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

  2. I. Introduction A. Excited electronic states • 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

  3. B. Fate of an isolated polyatomic system prepared in an excited electronic state • 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.

  4. C. Excited electronic states of molecular ions LUMO • 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. HOMO Hole states LUMO states 

  5. D. Theory of mass spectra 1. Quasi-equilibrium theory (QET) • Molecular ions in various electronic (and vibrational) states are produced by EI (or PI). • Ions in excited electronic states undergo rapid internal conversion to the ground state.  Rapid conversion of electronic energy to vibrational energy. • Intramolecular vibrational redistribution (IVR) occurs rapidly also.  Transition state theory, or, Rice-Ramsperger-Kassel-Marcus (RRKM) theory.  QET or RRKM – QET

  6. 2. Test • Prepare M+ with different E. • Measure or product branching ratios vs. E. • Compare with the calculated results. 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’

  7. II. Initial discovery A. Photodissociation of benzene cation Chopper Prism Argon ion laser Laser beam Lens J. Chem. Phys. 113, 9532 (2000) Electric sector Ion beam Electrode assembly Phase-sensitive detection 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. Magnetic sector PMT Laser beam R1 R3 R4 R5 R6 R7 R2 Ion source Collision cell Ion beam • 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.

  8. Electronic states ( C6H6+• ) Dissociation ( Products ) E(eV) 6 ~ E 2B2u ktot ~ 107s-1 5 ~ D 2E1u ktot ~ 104s-1 4 C6H6+•  C6H5+ + H• ~ 3 C 2A2u ~ B 2E2g 2 1 ~ 0 X 2E1g (ground state) Energy diagram of the benzene molecular ion. The lowest reaction threshold (E0) is 3.66 eV for C6H6C6H5H. ktot denotes the total dissociation rate constant in the ground state calculated from previous results. • For PD to be observed with the present apparatus, photoexcited C6H6+ must have E > 5 eV Remainder ? • Photon energy = 2.4 ~ 3.7 eV.

  9. PD-MIKE profile for the production of C4H4 from the benzene ion at 357nm 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 6.3107 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.

  10. 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 (5.51.1)107 and (53)106 s-1 for PDs at 357 and 488.0 nm, respectively, are marked. • 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

  11. 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+

  12. PD as a function of C6H6 pressure in the ion source Pressure dependences of the precursor (BZ) intensity (–––) and photoproduct (C4H4) intensities 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 number of collisions (Ncoll) suffered by ions exiting the ion source at some benzene pressures.

  13. 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 electronic state, C6H6 +†+C6H6 C6H6 +C6H6+† Population of C6H6 +† does not decrease by charge exchange.

  14. 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.

  15. Samples IE (eV) Low pressure High pressure 9.06 3.6 Chlorobenzene 3.5 Fluorobenzene 3.9 9.20 1.4 5.3 9.62 Benzonitrile 0.06 4.7 Chloropentafluorobenzene 9.72 0.01 3.8 Nitrobenzene 9.86 0.06 9.91 2.5 Hexafluorobenzene 0.02 10.51 3.0 Ethylene 0.02 4.4 11.32 Methylene chloride 0.04 4.7 Chloroform 11.37 0.03 11.47 3.4 Carbon tetrachloride 0.06 0.09 Ethane 11.52 ~0 0.05 Dichlorofluoromethane 11.75 0.04 11.98 0.16 1-chloro-1,1-difluoroethane 0.01 12.20 0.09 Chlorodifluoromethane 0.05 12.51 0.24 Methane ~0 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).

  16. 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 eV

  17. B. Summary Low-lying excited electronic states of C6H6+ IE = 9.243 eV IE = 12.3 eV IE =11.488 eV • 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)

  18. C6H6 Photoelectron Spectrum Sharp vibrational peaks for and .

  19. III. Charge exchange ionization to detect M+† J.Am. Soc. Mass Spectrom. 12, 1120 (2001). 1. Energetics A+ + B  A + B+, E , energy defect For A+ in the ground state, E > 0, endoergic = 0, resonant < 0, exoergic

  20. 2. Charge exchange cross section • 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

  21. 2) Charge exchange involving molecular species Exoergic charge exchange (E < 0) • 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. • For near thermal collision • large when E  0 • small when E > 0 Exoergicity rule

  22. 3. Instrumentation 1) Requirement Collision cell for conventional tandem mass spectrometry G Charge exchange • For charge exchange at low impact energy, M+ must be decelerated. • Should detect G+, which moves thermally inside the cell.  Low yield.

  23. 2) Instrumentation Second collision cell EM Ion beam Electric sector Conversion dynode Magnetic sector First collision cell PM Conversion dynode First collision cell Collision Cell Ion source Y-lens Ion Source Repeller

  24. 3) First Cell Ion source Collision cell Magnetic analyzer M+ Vs Vc 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

  25. 4) Second Cell Electrostatic analyzer Collision cell Ion source Magnetic analyzer  Vs Vc • 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) 

  26. 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 20s or longer.

  27. 2) First cell I I 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. I I I I I I III I

  28. 3) Relative yield of collision gas ions vs. impact energy ~ When A2E2g state is fully quenched RE ( C6H6+, ) = 9.243 eV

  29. ~ When A2E2g state is present RE ( C6H6+, ) = 11.488 eV

  30. 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.

  31. e- removal from 3b1  (3b1)-1 • 1a2  (1a2)-1 • 6b2  (6b2)-1 • 2b1  (2b1)-1 Hole states appearing in photoelectron spectra IV. Benzene derivatives J. Chem. Phys. In press, 2002. A. Halobenzenes 6b2 (Xnp∥ character) 2b1 (Xnp⊥ character) C6H6 C6H5X X 3b1 - 1e1g + - 1a2 + 6b2 2b1 np

  32. C6H5Cl Photoelectron Spectrum • Widths of vibrational bands of & are comparable.  Possibility of very long lifetime for of C6H5Cl+

  33. C6H5Br Photoelectron Spectrum • Widths of vibrational bands of & are comparable.  Possibility of very long lifetime for of C6H5Br+

  34. bands broader than  Rapid relaxation of of C6H5I+ C6H5I Photoelectron Spectrum

  35. (F2p∥)-1bands broader than  Rapid relaxation C6H5F Photoelectron Spectrum (F2p∥)-1

  36. Hole states appearing in photoelectron spectra B. Triple bonds C6H6 C6H5CN/ C6H5CCH C  X 3b1 1e1g 1a2 6b2 2b1  • 6b2  (CX∥) character 2b1  (CX⊥) character • e- removal from 3b1  • 1a2  • 6b2 

  37. C6H5CCH Photoelectron Spectrum C6H5CN Photoelectron Spectrum • Sharp vibrational bands for states. • Possibility of very long-lived states of C6H5CN+, C6H5CCH+.

  38. C. Experimental results 1) C6H5Cl+ C6H5Cl+( ) + CH3Cl  C6H5Cl + CH3Cl+ RE (C6H5Cl+, ) = 11.330 eV IE (CH3Cl) =11.28 eV E = 11.28 eV – 11.330 eV = -0.05 eV, exoergic! ~ CH3Cl+ would be observed if B of C6H5Cl+ is very long-lived.

  39. Partial mass spectrum of C6H5Cl generated by 20 eV EI recorded 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 collision 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+.

  40. 2) C6H5Br+ C6H5Br+( ) + CH3Br  C6H5Br + CH3Br+ RE (C6H5Br+, ) = 10.633 eV IE (CH3Br) =10.54 eV E = 10.54 eV - 10.633 eV = -0.093 eV, exoergic! ~ CH3Br+ would be observed if B of C6H5Br+ is very long-lived.

  41. Partial mass spectrum obtained under the single focusing condition with C6H5Br and CH3Br introduced into the ion source and collision cell, respectively. C6H5Br was ionized by 20 eV EI and acceleration energy was 4008 eV. Collision cell was floated at 3907 V.

  42. 3) C6H5CN+ C6H5CN+( ) + CH3Cl  C6H5CN + CH3Cl+ RE (C6H5CN+, ) = 11.84 eV IE (CH3Cl) =11.28 eV E = 11.28 eV – 11.84 eV = -0.56 eV, exoergic! ~ CH3Cl + would be observed if B of C6H5CN+ is very long-lived.

  43. Partial mass spectrum obtained under the single focusing condition with C6H5CN and CH3Cl introduced into the ion source and collision cell, respectively. C6H5CN was ionized 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 due to collision-induced dissociation of C6H4+to C4H2+ and C4H3+, respectively.

  44. 4) C6H5CCH+ C6H5CCH+( ) + CS2 C6H5CCH + CS2+ RE (C6H5CCH+, ) = 10.36 eV IE (CS2) =10.07 eV E = 10.07 eV - 10.36 eV = -0.29 eV, exoergic! ~ CS2+ would be observed if B of C6H5CCH+ is very long-lived.

  45. Partial mass spectrum obtained under the single focusing condition with C6H5CCH and CS2 introduced into the ion source and collision cell, respectively. C6H5CCH 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.

  46. Precursor ion Collision gas IE, eV C6H5Cl+•C6H5Br+•C6H5CN+•C6H5CCH+•C6H5I+•C6H5F+• (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 ~ 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* ~ Collision gases, their ionization energies(IE) in eV , and success / failure to generate their ions by charge exchange with some precursor ions

  47. ~ ~ ~ Recombination energies of the X2B1, A2A2, and B2B2 states and the oscillator strengthsof the radiative transitions from the B2B2 states. ~ C6H5Cl+ C6H5Br+ C6H5I+ C6H5CN+ C6H5CCH+ State ~ 9.066 (0.0000000) X2B1 8.991 (0.0000000) 8.754 (0.0000000) 9.71 (0.0000000) 8.75 (0.0000000) ~ 9.707 (0.0000008) A2A2 9.663 (0.0000001) 9.505 (0.0000000) 10.17 (0.0000010) 9.34 (0.0000004) ~ B2B2 11.330 10.633 9.771 11.84 10.36 13.236 Lowest quartet 13.381 13.3 12.7 12.664 11.891 12.356 12.725 11.07 Reaction threshold 12.41 ~ • 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. ~ ~ ~ ~

  48. V. Vinyl derivatives • 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 Magnetic analyzer Electrostatic analyzer Ion source Collision cell Vs Vc 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.

  49. a ( Xnp∥ character) a ( Xnp⊥ character) C2H4 C2H3X X a  a Xnp a Hole states appearing in photoelectron spectra B. Vinyl halide • e- removal from a (C=C)  • a (Xnp∥)  • a (Xnp⊥) 

  50. 1) Vinyl chloride • Sharp vibrational bands for •  Possibility of very long lifetime.

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