The Pure Rotational Spectrum of TiCl + (X 3  r ) by Velocity Modulation Spectroscopy DeWayne T. Halfen and Lucy M. Ziurys Department of Chemistry Department of Astronomy Steward Observatory Arizona Radio Observatory University of Arizona June 21, 2004

Why Molecular Ions ? Chemical Aspect –Organic intermediates –Stratospheric chemistry –Combustion chemistry Astronomical Aspect –Many molecular ions in interstellar medium Radicals – Difficult to study –Few molecular ions studied at high resolution Ions Studied at High Resolution H3+H3+ CO + CH + SO + HCO + HOC + H2D+H2D+ HCS + N2H+N2H+ H3O+H3O+ CH 2 D + HCNH + HOCO + H 2 COH + HC 3 NH + OH + OH  HCl + ArH + HBr + Neutral species dominate over ions Built a new spectrometer – uses Velocity Modulation –Ion selective technique

Velocity Modulation Spectrometer

Detector Radiation Source Gas Cell Reactant

Characteristics of Velocity Modulation Drift velocity of TiCl + – –E = 1.9 V/cm,  (Ar) = Å 3,  (Ar-TiCl + ) = amu, T = 208 K, P = 50 mTorr –v d = m/s Doppler shift – –  = 606 kHz at v 0 = 396 GHz Line width:  ~ 1300 kHz Modulation index =  /  = 0.46 –Under-modulated at millimeter wavelengths

Source Modulation Velocity Modulation Source Modulation Velocity Modulation Source Modulation Velocity Modulation <5% leakage of neutral signals in VM mode Using Velocity Modulation Use velocity modulation to distinguish neutrals from ions

Past Studies of TiCl + Balfour & Chandrasekhar (1990) –First observed visible spectrum Kaledin & Heaven (1995,1997a,1997b) –Predicted X 3  r & confirmed using laser absorption Focsa et al. (1997a,1997b,1998,1999) –Used laser absorption/velocity modulation to measure several electronic transitions –Found that the  = 2 & 3 subbands of X 3  r perturbed by  3  r state –Established spectroscopic constants for each state

Gas-Phase Synthesis of TiCl + Add TiCl 4 –Pressure: <1 mTorr 20 mTorr Ar gas also added AC discharge –200 W at 600 

Energy Level Diagram for TiCl + 3  r ground state –Two unpaired 3d electrons J = L + S Spin-orbit and spin-spin interactions – & Omega ladders –  = 2, 3, 4 – J ≥  A 3  r state close in energy –Perturbs  = 2 & 3 ladders

Source Modulation Velocity Modulation Rotational Spectrum of TiCl + Fine structure components shifted from normal pattern TiCl + confirmed by VM –S/N down by factor of 4 Measured 37 Cl, 46 Ti isotopomers in natural abundance

Measured 10 rotational transitions of 48 Ti 35 Cl +, 48 Ti 37 Cl +, & 46 Ti 35 Cl + JJ J  obs obs - calc JJ J  obs obs - calc 30           Rest Frequencies of 48 Ti 35 Cl + (X 3  r )

Spectroscopic Analysis of TiCl + Determined spectroscopic constants for TiCl + 48 Ti 35 Cl + ParameterMMWOptical B (21) (54) D (78) (45) A a (80) ADAD (89)-0.489(54) AHAH 6.308(36) x (5000)11030(180) D (46)0.507(18) H x a rms0.037 a Held Fixed Rotational constants agree Fine structure parameters different –Different analysis methods reflects large perturbation from A 3  r state

TiCl vs. TiCl + Relative intensities of TiCl vs. TiCl + very similar Neutral only ~1.5x stronger than ion Ions usually very small fraction of plasma TiCl 4 produces ions well

Future Work Measure spectra of more titanium species & molecular ions –TiC (X 3   ), TiN + (X 1  + ), TiO + (X 2  r ), TiF + (X 3  r ) VCl - TH11 VCl + - TH12