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ItemElectronic spectroscopy of gas-phase ions( 2018)This dissertation is centered around the measurement and analysis of the gas-phase electronic spectra of small carbocations and ionic complexes. These electronic spectra are obtained by photodissociating mass-selected cations and rare gas complexes in a custom built tandem mass spectrometer. The species studied include the propargyl (H2C3H+), methyl propargyl (H2C4H3+), and protonated diacetylene (H2C4H+) cations, and the Ar-O2+ and N2-O2+ complexes. The spectroscopic studies of cold molecular cations and ionic complexes described in this thesis impart a clearer understanding of their vibronic structure, providing the spectroscopic data necessary for their possible detection in interstellar, combustion, and plasma environments. The electronic spectrum of H2C3H+ is measured over the 230-270 nm range by photodissociating mass-selected Ne- and N2-tagged complexes. The band system is assigned to the B1A1<-X1A1 transition, and experimental band origins are observed at 37 618 cm-1 for H2C3H+-Ne and 37 703 cm-1 for H2C3H+-N2. The ground and excited states of H2C3H+ are characterized using ab initio restricted active space self-consistent field (RASSCF) and coupled-cluster (CC) response methods. These computational studies show that H2C3H+ has C2v symmetry in both the ground and excited state, and the main extended vibronic progression -- experimental spacing of 630 cm-1 -- is assigned to the C-C stretch vibrational mode, ν5. Electronic spectra for the H2C4H3+ cation are observed over the 230-270nm range and the band system is assigned to the B1A'<-X1A' transition. These spectra are measured by photodissociating the bare cation and its Ar- and N2-tagged complexes, mainly forming C2H3+ + C2H2 (protonated acetylene+acetylene) or H2C4H+ + H2 (protonated diacetylene+dihydrogen). Experimental band origins occur at 37 753 cm-1 for H2C4H3+, 37 738 cm-1 for H2C4H3+-Ar, and 37 658 cm-1 for H2C4H3+-N2. Each spectrum displays similar vibronic structure with main progression experimental spacings of 630 cm-1. The main progression is assigned to the symmetric C-C stretch vibrational mode, ν11, based on comparison with higher order computations reported for the structurally similar H2C3H+ cation, and ground and excited state calculations of H2C4H3+ using time-dependent density functional theory (TD-DFT) at the wB97X-D/aug-cc-pVDZ level of theory. The dissociation pathways of H2C4H3+ on the ground state manifold are elucidated using ab initio calculations and master equation simulations. Agreement is found between experimental and simulated product branching ratios favoring acetylene elimination, with channel switching at higher energies to H2 elimination. The B1A1 <- X1A1 electronic band system of the H2C4H+ cation is measured over the 230-295 nm range by photodissociating H2C4H+ tagged with Ar and N2 in a tandem mass spectrometer and by photodissociating bare H2C4H+ ions stored in a cryogenic ion trap. The band origin occurs at 34 934 cm-1 for H2C4H+-Ar, 34 920 cm-1 for H2C4H+-N2 and 34 941 cm-1 for bare H2C4H+ and each spectrum displays similar well resolved vibronic structure. Second-order M οller-Plesset perturbation (MP2) and second-order approximate CC (CC2) ab initio calculations are used to assign the observed vibronic progressions to two symmetric C-C stretch vibrational modes ( ν6 and ν4), with experimental band spacings of 860 and 1481 cm-1, respectively. Vibronic transitions for the Ar-O2+ and N2-O2+ ionic complexes are recorded over the 230-350 nm range, which includes wavelength regions in which the respective atomic and diatomic fragments, O2+, O2, Ar+, Ar, N2+, and N2 do not have recognized electronic transitions. The Ar-O2+ and N2-O2+ band onsets correspond closely to the difference in the ionization potentials of Ar and O2, and N2 and O2, respectively, consistent with intermolecular charge transfer transitions. The dominant vibronic progressions have spacings of 1565 cm-1 for Ar-O2+ and 1449 cm-1 for N2-O2+, similar to the vibrational frequency of the neutral O2 molecule in its O2 X 3 Σg- state, further suggesting the transitions involve an electron transfer from Ar or N2 to O2+. The Ar-O2+ and N2-O2+ complexes are characterized using ground state CC methods, which further aids in the ιnterpretation of the observed vibronic structure. The first documented gas-phase ultraviolet electronic spectra of these carbocations and ionic complexes are presented in this dissertation. Each spectrum shows well resolved vibronic structure for clear comparisons with electronic structure calculations. Although the measured spectra are well resolved, these studies will hopefully provide useful data for further high-resolution spectroscopic studies that may lead to the detection of these molecular species in extraterrestrial, combustion, and plasma environments.