Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton ionization of barium with a tunable pulsed laser in the range 570-608 nm

Sanjay G. Nakhate, S. A. Ahmad, M. A. N. Razvi, G.D. Saksena

Published Dec 14, 1991• 12 citations

Abstract

Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm -2 . Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba + . The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 10 13 atoms/cm 3 ) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2 P 1/2,3/2 ), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2 P 3/2 state relative to the 6p 2 P 1/2 state.

DOI: 10.1088/0953-4075/24/23/023

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Journal of Physics B Atomic Molecular and Optical PhysicsFeb 14, 198926 citations

Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton ionization of barium with a tunable pulsed laser in the range 570-608 nm

Sanjay G. Nakhate, S. A. Ahmad, M. A. N. Razvi, G.D. Saksena

Published Dec 14, 1991• 12 citations

Publisher

Abstract

DOI: 10.1088/0953-4075/24/23/023

Related papers

Fluorescence studies of multiple-photon ionization processes: Four- and five-photon ionization of Sr at wavelengths of 558–590 nm

Harold K. Haugen, Andreas Othonos

An ``all-optical'' technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (\ensuremath{\sim} ${10}^{11}$ -- ${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558--590 nm at laser intensities \ensuremath{\sim}1--50 GW/ ${\mathrm{cm}}^{2}$ . Fine-structure-resolved emission of the ${\mathrm{Sr}}^{+}$ (5 ${p}^{2}$ ${P}_{3/2/,1/d}^{0}$ ) ions which are formed is observed at \ensuremath{\lambda}=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity \ensuremath{\sim} ${1:10}^{3}$ with respect to the ionic-resonance emission.An ``all-optical'' technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (\ensuremath{\sim}${10}^{11}$--${10}^{15}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558--590 nm at laser intensities \ensuremath{\sim}1--50 GW/${\mathrm{cm}}^{2}$. Fine-structure-resolved emission of the ${\mathrm{Sr}}^{+}$ (5${p}^{2}$${P}_{3/2/,1/d}^{0}$) ions which are formed is observed at \ensuremath{\lambda}=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity \ensuremath{\sim}${1:10}^{3}$ with respect to the ionic-resonance emission.

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Multiphoton double ionization of barium with intense picosecond pulses

Duncan Tate, Dimitrios G. Papaioannou, Thomas F. Gallagher

The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.The authors have observed the production of Ba2+ from neutral Ba by intense picosecond laser pulses in the range 560-650 nm. Several new features have been observed, which were not observed with nanosecond pulses. Most of the strong resonances in Ba2+ production can be attributed to Ba+ transitions originating in the 6s1/2 or 5dj states, suggesting that the dominant process in double ionization in this intensity and wavelength regime is sequential removal of electrons. However, there are several resonances which cannot be attributed in this manner, evidence for possible alternative ionization mechanisms. Ba2+ production is strong throughout the region studied, at times comparable to that of Ba+. The authors have also looked for Ba2+ production in the range 280-325 nm, but no Ba2+ signal was observed in this part of the ultraviolet.

Journal of Physics B Atomic Molecular and Optical PhysicsApr 28, 199113 citations

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Harold K. Haugen, Henrik Stapelfeldt

Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from ${\mathrm{Ca}}^{+}$ (4p), ${\mathrm{Ca}}^{+}$ (5s), and ${\mathrm{Ca}}^{+}$ (4d), and from ${\mathrm{Sr}}^{+}$ (5p) and ${\mathrm{Sr}}^{+}$ (6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from ${\mathrm{Ca}}^{+}$(4p), ${\mathrm{Ca}}^{+}$(5s), and ${\mathrm{Ca}}^{+}$(4d), and from ${\mathrm{Sr}}^{+}$(5p) and ${\mathrm{Sr}}^{+}$(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.

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Multiphoton single and double ionisation of strontium in the range 532-541 nm

Pierre Camus, Μιχάλης Κομπίτσας, S. Cohen +4 more

By focusing an excimer pumped dye laser on an atomic beam, the authors have observed multiphoton multiple ionisation of the neutral strontium atom in the tuned wavelength range 532-41 nm. Sr + and Sr ++ yields were measured with a time of flight spectrometer. At least three photons are required to produce Sr + and eight photons to produce Sr ++ . As a function of the wavelength, the Sr + yield exhibits resonances which correspond to two-photon excitation of bound states (5s 2 -5p 2 1 D 2 and 5s 2 -5p 2 1 S 0 ) and three-photon excitation of autoionising states 4d5f J=1 and J=3 of the neutral atom. The Sr ++ yield exhibits four broad smooth resonances that they interpret, from known spectroscopic data, as two-photon (5p 1/2 -4f), three-photon (5s-6p 1/2, 3/2 ) and four-photon (5s-7d) resonances in the singly charged ion. The latter transitions start from a singly charged ion in the 5s or 5p 1/2 state reached after three- or four-photon ionisation of the neutral ground state. The variation of the Sr ++ pattern as a function of the dye laser beam intensity shows that resonances are shifted and broadened when the intensity is increased. These features have been interpreted by a perturbative treatment of Sr + ionisation based on the use of an effective Hamiltonian which takes into account higher-order terms involved in the calculation of resonance effects. Calculated multiphoton ionisation probabilities are in good agreement with experimental data.By focusing an excimer pumped dye laser on an atomic beam, the authors have observed multiphoton multiple ionisation of the neutral strontium atom in the tuned wavelength range 532-41 nm. Sr + and Sr ++ yields were measured with a time of flight spectrometer. At least three photons are required to produce Sr + and eight photons to produce Sr ++ . As a function of the wavelength, the Sr + yield exhibits resonances which correspond to two-photon excitation of bound states (5s 2 -5p 2 1 D 2 and 5s 2 -5p 2 1 S 0 ) and three-photon excitation of autoionising states 4d5f J=1 and J=3 of the neutral atom. The Sr ++ yield exhibits four broad smooth resonances that they interpret, from known spectroscopic data, as two-photon (5p 1/2 -4f), three-photon (5s-6p 1/2, 3/2 ) and four-photon (5s-7d) resonances in the singly charged ion. The latter transitions start from a singly charged ion in the 5s or 5p 1/2 state reached after three- or four-photon ionisation of the neutral ground state. The variation of the Sr ++ pattern as a function of the dye laser beam intensity shows that resonances are shifted and broadened when the intensity is increased. These features have been interpreted by a perturbative treatment of Sr + ionisation based on the use of an effective Hamiltonian which takes into account higher-order terms involved in the calculation of resonance effects. Calculated multiphoton ionisation probabilities are in good agreement with experimental data.

Journal of Physics B Atomic Molecular and Optical PhysicsFeb 14, 198926 citations

{10}^{11}

{10}^{15}

${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558--590 nm at laser intensities \ensuremath{\sim}1--50 GW/

{\mathrm{cm}}^{2}

. Fine-structure-resolved emission of the

{\mathrm{Sr}}^{+}

{p}^{2}

{P}_{3/2/,1/d}^{0}

) ions which are formed is observed at \ensuremath{\lambda}=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity \ensuremath{\sim}

{1:10}^{3}

with respect to the ionic-resonance emission.

We have investigated multiphoton ionization of Ba

6snl

Rydberg states by intense 200-fs laser pulses. Specifically, we have excited Ba atoms to the

6sns

and

6snd

15\ensuremath{\infty} states using two nanosecond lasers. The atoms were then irradiated with the fs pulses produced by a mode-locked, regeneratively amplified Ti:sapphire laser that is stepwise-tunable between 790 and 850 nm. We have used both the fundamental light and the second harmonic (395--425 nm) to investigate the ionization dynamics in these two wavelength regions. In contrast to previous experiments, in which laser pulses 5 ps or longer were used, our results indicate that excitation of

{\mathrm{Ba}}^{+}

Rydberg states and

{\mathrm{Ba}}^{2+}

photoions is relatively insensitive to the femtosecond laser wavelength. In addition, we find that, as the Kepler orbital period of the Ba Rydberg electron becomes comparable to this very small laser pulse width, the inner electron ionization process no longer appears to be instantaneous to the outer electron, and the outer electron is not projected onto

{\mathrm{Ba}}^{+}

Rydberg states.

Multiphoton ionization of strontium in the wavelength range of \AA{}558-590 nm is studied via the fluorescence from excited

{\mathrm{Sr}}^{+}(5p^{2}P_{\frac{3}{2},\frac{1}{2}})

ions. The fine-structure sublevels in the singly charged ion are fully resolved and are found to be nonstatistically populated. Four- and five- (or more) photon processes are observed to populate

{\mathrm{Sr}}^{+}(5p^{2}P)

in the intensity regime of $I\ensuremath{\sim}1\ensuremath{-}50$ GW/

{\mathrm{cm}}^{2}

, but no emission is detected from higher-lying states of

{\mathrm{Sr}}^{+}

{\mathrm{Ca}}^{+}

(4p),

{\mathrm{Ca}}^{+}

(5s), and

{\mathrm{Ca}}^{+}

(4d), and from

{\mathrm{Sr}}^{+}

(5p) and

{\mathrm{Sr}}^{+}

(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.