Two Dimensional Coherent Double Resonance Electronic Spectroscopy Ohio State University Molecular Spectroscopy Conference June 20, 2008 Peter C. Chen Department of Chemistry Department of Chemistry Spelman College Atlanta GA

Rotational congestion A common challenges when interpreting high resolution molecular spectra A common challenges when interpreting high resolution molecular spectra More difficulty with increasing size of the molecule More difficulty with increasing size of the molecule More difficult to resolve peaks More difficult to resolve peaks More difficult to interpret the spectra More difficult to interpret the spectra Increased chance of perturbations (e.g., conical intersections) Increased chance of perturbations (e.g., conical intersections) Increased chance of mixtures (e.g., isotopomeric or conformational mixtures) Increased chance of mixtures (e.g., isotopomeric or conformational mixtures)

High Resolution Coherent Two Dimensional Spectroscopy On-diagonal peaks: Similar to 1D spectroscopy Off-diagonal peaks: Provides new information: coupled transitions leads to coupled Fortrat parabola clusters. Benefits: not only improved spectral resolution but also peaks sorting (by rotational and vibrational quantum numbers, selection rules, isotopomer, etc.) I or A  or  4 1 2D Contour Plot (data is a 3D surface) Conventional Spectrum (data is in 2D) Four wave mixing CDRESRECARS Coupled transitions

Sample: C 2 a 3  u ← d 3  g Reference: P.C. Chen and C.C. Joyner, Analytical Chemistry 77, , 2005 simulation Coupled Fortrat Parabola! experiment RECARS

Sample: I 2 B 3  u ← X 1  g transition 550 nm564 nm 550 nm564 nm Conventional 1D absorption spectrum: Coherent 2D spectrum: Peaks appear more organized: separated and sorted by…. Typical rotational congestion: peaks from different vibrational transitions are overlapped CDRES

550 nm v 4 ’=23 v 4 ’=22 v 4 ’=21 v 4 ’= nm nm nm 1 (nm) 4 (nm) Sample: Iodine vapor v 1 ’=16 v 1 ’=15 Reference: P. C. Chen and C. C. Joyner, JPC A 110, , Peaks are sorted by vibrational quantum number… v 1 ” = 0 to… v 4 ” = 0 to… CDRES

…and sorted by rotational quantum number Green arrows indicate J”=87 (J’=86 and 88) for the v”=0, v’= 23 parabola. (  J=+1 different location from  J=-1). PEAKS SORTED BY ROTATIONAL QUANTUM NUMBER AND SELECTION RULE. By comparison, peaks appear disordered in conventional (1D) spectra. simulation: J”=1 to 90 V”=0 V’ 4 =19 to 23 V” 1 =15 to 16  4 → → J”=0 0”→23’0”→22’ (peaks sorted by vibrational quant. #) Experimental Simulated v 4 =

Instrument: 1.25 meter monochromator w/ CCD Pixel spacing: ~0.01nm or 0.4 cm-1 CDRES spectrum of Br 2 (g) B 3  u + ← X 1  g + (nm) Absorbance 513 nm 514 nm 508 nm 528 nm Peak markers: red= 79 Br 2, blue= 81 Br 2, green= 79,81 Br 2 ; J=1-198; v=0-50 Peak density: approximately 1000 peaks per nm Spectral resolution needed: sub-picometer (Doppler broadening ~0.4 pm).

81 Br 2 79,81 Br 2 79 Br 2 Br 2 peaks separated and sorted by isotopomer Note: Experimental results show some effects from lower lying parabolas, (v 3 ’>25) 4 (nm) 1 (nm) v 4 =26 v 4 =27 v 4 =28v 4 =29 v 3 =25 Reference: P. C. Chen and M. Gomes, JPC A 112, , 2008.

Summary High resolution coherent 2D techniques like CDRES can –Improves spectral resolution –Sorts peaks by vibrational and rotation quantum numbers –Sorts peaks by selection rule –Sorts peaks by isotopomer Capabilities demonstrated using C 2, I 2, and Br 2 (diatomics with well-characterized spectra). What about something trying something more complicated (e.g., not well-characterized)?

Conventional spectrum of NO 2 an important photochemical pollutant –Broad (absorbs from IR to UV) –Lacks regular patterns Visible absorption spectrum first observed: D. Brewster, Trans. R. Soc. 12, 519, 1834

NO 2 ‘s notoriously difficult spectrum Jet spectra 1,2 show unusually high density of irregularly spaced vibronic levels. (~350 vibrational origins observed to be distributed chaotically over the range nm. Average spacing: 10 cm -1 ). Low-lying conical intersection (at ~10,000 cm -1 ) causes strong vibronic mixing (hybrid states) with spacings that appear chaotic. Rotational spacings are also irregular. Result: severe rotational congestion; very few regions assigned. 1. R.E. Smalley, L. Wharton, and D.H. Levy, JCP 63, 4977, R. Georges, A. Delon, and R. Jost, JCP 103, 1732, 1995.

–Broad (absorbs from IR to UV) –Lacks regular patterns 1 4 Conventional spectrum of NO 2 an important photochemical pollutant

Observation: Lots of X’s, arranged in an irregular pattern 561 nm579nm 600 nm 590 nm 1 4 CDRES spectrum of NO 2

Simulation This simulation shows the expected location of the most intense coupled vibronic transitions. Values used in the simulation are based upon previously measured vibrational origins. Coherent 2D spectrum of NO 2 Therefore, the X’s in the 2D spectrum mark the locations of vibronic origins

R,RR,R P,PP,PR,PR,P P,RP,R N” N’ = N” Note: the N”=0 peak is found in the center. N” increases systematically with increasing distance from the center. (for the K=0 stack of NO 2, N”=0, 2, 4, 6, etc.) X-shaped cluster occurs when B’ = B” P = N” to N’-1 R = N” to N’+1 Explanation of the X-shaped structure

2D plot of relevant peaks above 2500 cts 4 1 Expand one of the X’s: resulting pattern more complicated

2D plot of relevant peaks above 2500 cts 4 1

Fine splitting due to spin-rotation interaction Multiple vibrational origins Combined effect 1 4

4 1 2D plot of relevant peaks above 2500 cts N”=4 N”=6 N”=8 N”=10 N”=12

Results Results for previously explored isolated vibrational origin near nm match prior values 1,2 : B’ = (+0.004) cm-1 Results for the two (previously not analyzed) overlapping vibrational origins near nm are: B’ = (+0.004) cm -1 for the vibrational origin at cm -1 ) B’ = (+0.004) cm -1 for the vibrational origin at cm -1 ) Future goal: can we use this approach to perform a complete rotational analysis for the entire NO 2 electronic spectrum? B’ = (B’+C’)/2 Near-prolate symmetric rotor: 1. C.G. Stevens and R.N. Zare, JMS 56, 167, T. Tanaka, R.W. Field, and D.O. Harris JMS 56, 188, 1975

Students: Marcia Gomes Lindsai Bland Kamilah Mitchell Simone Colbert Collaborators: Michael Heaven, Emory University Charles Hardnett, Spelman College Funding: NSF grants CHE and EEC Acknowledgements