The Wittig Group

Hydrocarbon Radicals

The C2H radical is an excellent prototype for ex­am­­ining important phenomena: electronic states; curve crossings and nonadia­batic transitions; in­tra­molecular and disso­cia­tion dynam­ics; and so on. It is small enough to provide ex­peri­mental parent and product state resolution, and it is tractable at a high level of theory: both el­ec­tronic struc­ture and quantum me­chanical nuclear dy­nam­­ics, including non­adia­batic couplings. It is central to a large body of chemis­try, and a number of coni­cal intersections need to be taken into ac­count sim­ul­taneously. Its com­plex­ities are ad­van­tageous, as they derive from processes that are difficult to un­ravel in larger ra­di­cals. Because C2H is small, trac­table at a high level of theory, and rife with cross­ings, non­adi­a­­batic coup­lings, etc., it pro­vides a unique platform.

Conical intersection

Con­­­­i­cal in­ter­­sec­tions lead to subtle ef­fects. For ex­ample, con­sider the coupled  sys­tem at en­ergies near 32A’. Because 12A’ and 22A’ in­ter­sect con­i­cally and remain coup­led X᷉/A above their in­ter­­sec­tion, the 32A’/ 22A’ in­ter­sec­tion re­sults in a man­­ifold in which  and  char­acters are pre­sent in wave functions. This pro­vi­des ac­cess to ground state pro­ducts.

Experimental strategy

To characterize the C2H molecular beam, a TOF mass spectro­meter samples ex­pan­ded gas at intervals as short as 5 μs (see figure on the right) This diagnostic has also been used in other ex­­per­i­ments in our laboratory with great success. For example, con­den­sed H2O/CO2 is ab­la­ted using an IR pulse, and TOF mass spectra are recorded. The figure below shows one such TOF spectrum. The lar­gest peaks are off-scale; the most intense has S/N of ~ 103.

Referring to the above figure, tunable UV will be directed to the region of the focused e-beam. It will overlap the e-beam and be synchronized to a given TOF spectrum. Yield spectra will be ob­tain­ed by moni­toring C2+ as the frequency is scanned. This signal arises from dis­so­ciation: C2H → C2 + H , which is known to dominate (i.e., over fluor­es­cence decay of electronically excited C2H) through­out a large spectral range. Short residence times can be achieved by pho­tolyzing samples at the end of the tube. With 193 nm radia­tion fo­cused with a cyl­in­drical lens such that it intercepts a region of 2 mm length at the end of the tube, C2H pulse durations of 10-20 μs are achiev­ed.

Dynamics

Assume that ground state C2H is photodissociated at ~ 240 nm and the avail­able en­ergy is 2500 cm–1. The figure shows C2 vibrations and rotations for  and . The 718 cm-1 separation be­tween X1Σ+g and , a3Πμ, as well as vi­bra­tions, are easily resolved, and even rotational levels will be re­­solved for J greater than ~ 8. Be­cause HRTOF records an entire spectrum with each laser firing, there is no distortion across the TOF range. Thus, relative populations are ob­tain­ed with good accuracy.

© 2010-2013 Curt Wittig