J. CHEM. SOC. FARADAY TRANS., 1992, zyxwvutsrq 88(4), 525-529 525 Observation of Perturbations in the Rotational Manifold of the CN B zyxwvuts *C+ zyxwvu Y zyxwvutsrq = 1 Level caused by Interaction with the CN A zy 211. Y = 12 Level I J. F. Black7 and R. N. Zare* Department of Chemistry, Stanford University, Stanford, California 94305-5080 USA The (1, 0) band of the CN B 2C+-X 2C+ transition has been examined using sub-Doppler laser-induced fluores- cence (LIF), and the spin-rotation splitting of the upper state was found to vary irregularly. This behaviour is identified as arising from localised perturbations of the CN zyxwv 6 'C+ (v = 1) level caused by interaction with levels of the same total angular momentum arising from the v = 12 vibrational level of the A2ni state. A deperturba- tion analysis based on diagonalising the Hamiltonian of the B 'C+ and A zyxwvu 211i states including their mutual inter- action is able to match well the experimentally observed separations between the F,(J = N + +) and f2(J = N - 4) levels of the B 2C+ state. The CN B2C+-X2Z+ system (the violet band system) has been the subject of intense scrutiny for many years.'-9 The transition itself has become a classic textbook example of a 2C2X transition while extra interest stems from the highly visible and theoretically tractable interaction of the levels of the B 2C+ state with high-lying rovibrational levels of the A 2ni state. The CN B-X transition has also found favour as a probe of the internal quantum state distribution of CN (X 2C+) rad- icals formed in a variety of chemical dynamics experiments. In particular, the photodissociation of the family of cyanogen halides has become a benchmark against which theories are tested and new experiments Our own interest here stems from the photodissociation process ICN + hv(249 nm) -, CN X 2C+(u, N) + I(2P1/2,3,2) which has been under extensive study in this laboratory for some Of particular interest are the ongoing efforts to understand the non-adiabatic forces acting in the disso- ciation, which manifest themselves in the most detailed experimental studies. In the course of making these measure- ments on the (1, 0) band of the B-X system, we have identi- fied a perturbation in the rotational manifold of the u = 1 level of the B state. Our experiments are sensitive to the spin- rotation splitting of the N levels in the B2X+ state, and we observe the perturbation as a non-monotonically varying value of the effective spin-rotation doubling constant yLff for the u = 1 level of the B state. The experimental values of the F,(J = N + 3) and F2(J = N - zyxw 3) spin-rotation doublet level separation obtained from these values of yLff are compared with those calculated by diagonalising the Hamiltonian matrix representing the 2C-211i interaction. The existing body of spectroscopic data on the CN A-B interaction is found to reproduce well the effects observed here. Experimental Fig. 1 shows a schematic diagram of the experimental appar- atus, which has been discussed extensively elsewhere.20 Briefly, the output of an excimer laser (Lumonics TE 860-2) is divided into two equal parts. One beam is recollimated, irised down and polarised to form the photolysis beam. This is SERC/NATO post-doctoral fellow 1987-1989. Present address: Department of Chemistry, Columbia University, New York, NY 10027, USA. directed into the vacuum system via extensively baffled arms, which are also equipped with an argon purge. This purge helps counter the build-up of photolysis debris on the windows and serves to reduce absorption of the 249 nm photons by the ICN before the scattering region is reached. Typical powers are d850 pJ per pulse in an 8 mm diameter spot. The other excimer beam pumps a dye laser (Lambda Physik FL2002E) operating at zyxw ca. 358 nm or greater (PBD dye, Exciton in cyclohexane). This wavelength is resonant with the (1, 0) transition in the B-X system.20 The beam is sent round an optical delay line, is polarised and then directed into the vacuum system uia extensively baffled arms. The pulse arrives at the scattering centre ca. 35 ns after the photolysis pulse. We observe the nascent CN photofragment distribution at partial pressures of Ar (10 mTorr) and ICN (2-10 mTorr). Typical powers for the dye laser are 100-400 nJ per pulse in a 6 mm diameter spot. The photolysis and probe beams are monitored by photo- diodes (Hamamatsu S1336-5BQ) coupled to a sample-and- hold circuit built in-house. An etalon (Virgo ET-100) is used to monitor the dye laser linewidth and provide a standard against which to linearise the frequency abscissae of the sub- Doppler spectra generated. The dye laser is pressure-tuned using filtered dry nitrogen. LIF photons are collected along the direction normal to the plane containing the photolysis- and probe-beam wave- vectors. A single lens (1.5" diameter, f= 60 mm) in the vacuum chamber collects the fluorescence and focuses it onto the photocathode of a photomultiplier tube (RCA 7326). The output is amplified and digitised for ~120 ns (ca. 2 lifetimes'). Data are normalised to the pump and probe laser powers. ICN (Kodak) flows through the chamber as a vapour (2-10 mTorr). There is no evidence of memory effects from previous laser pulses or impurities. Results Broad-band Spectra Fig. 2 shows a portion of the fluorescence excitation spec- trum of the R branch of the B-X (1, 0) band. The minor fea- tures are the R branches of the (2, l), (3, 2) and (4, 3) bands of the same electronic system. The eye is immediately drawn to the perturbation at N" = 45, evidenced by the complete resolution of the two spin-rotation doublets of this line. This spectrum was taken under broad-band conditions, with no etalon in the dye laser cavity. Published on 01 January 1992. Downloaded by Stanford University on 21/09/2013 19:51:58. View Article Online / Journal Homepage / Table of Contents for this issue