A two-centre model is employed to study the electronic structure of argon plasma at a density of 1022 cm^-3 and a temperature of 5 eV. The model takes into account the influence of the nearest neighbour on the electro...A two-centre model is employed to study the electronic structure of argon plasma at a density of 1022 cm^-3 and a temperature of 5 eV. The model takes into account the influence of the nearest neighbour on the electronic structure of the radiator and gives a proper description of the transient molecular behaviour in dense plasmas where the mean interatomic distance can be the order of the orbital spatial extent. The orbital energies of quasi- molecular Ar2 against the interatomic distance are calculated in the framework of density functional theory. It is shown that the orbital energies are not monotonic functions of the distance because of two competitive effects： the repulsive potential between electrons and the attractive potential between electrons and nuclei. The chaxaeteris^ics of this two-centre system can be categorized by three regimes depending on the ratio between the electron wavelength and the interatomic distance： atomic regime, screened atomic regime, and quasi-molecular regime. This classification agrees qualitatively with the conclusion of full quantum-mechanical approach [Yonger S Met al 1988 Phys. Rev. Lett. 61 962].展开更多
Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually,thereby greatly enhancing the scope of Stark decelerated molecules.For comparison,we detect decelerated and trapp...Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually,thereby greatly enhancing the scope of Stark decelerated molecules.For comparison,we detect decelerated and trapped ammonia molecules using two different schemes:(ⅰ) nonresonant multiphoton ionization using intense femtosecond(fs) pulses in the near infrared,and(ⅱ) resonance-enhanced multiphoton ionization using nanosecond(ns) pulses from a tunable UV laser.The observed number of ions per shot for both schemes is similar.The fs laser detection scheme suffers from an increased background,which can be effectively eliminated by subsequent mass and velocity selection.To determine the detection volume of the ns laser detection scheme,we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below~100 μm.It is concluded that the detection volume for the ns laser detection scheme is 1.5-2 times larger than that of the fs laser detection scheme.展开更多
We experimentally demonstrate third-harmonic generation(THG)in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic(SH).The mechanism of THG has been investigated,and it demonstrates that a t...We experimentally demonstrate third-harmonic generation(THG)in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic(SH).The mechanism of THG has been investigated,and it demonstrates that a third-order nonlinear process dominates at low pump intensity.Asymmetric third-harmonic(TH)spectra are observed at different time delays in two color fields,which are attributed to the process of the four-wave mixing(FWM)of the broad spectrum of pump pulses.A joint measurement on the terahertz(THz)and the TH is performed.It reveals that the optimized phase for the THG jumps from 0 to 0.5πas the pump intensity increases,which is different from the THz being a constant,and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.展开更多
基金Supported by the National Natural Science Foundation of China under Grant No 10676039, the National Basic Research Programme of China under Grant No 2007CB815105, and the National High-Tech ICF Committee in China.
文摘A two-centre model is employed to study the electronic structure of argon plasma at a density of 1022 cm^-3 and a temperature of 5 eV. The model takes into account the influence of the nearest neighbour on the electronic structure of the radiator and gives a proper description of the transient molecular behaviour in dense plasmas where the mean interatomic distance can be the order of the orbital spatial extent. The orbital energies of quasi- molecular Ar2 against the interatomic distance are calculated in the framework of density functional theory. It is shown that the orbital energies are not monotonic functions of the distance because of two competitive effects： the repulsive potential between electrons and the attractive potential between electrons and nuclei. The chaxaeteris^ics of this two-centre system can be categorized by three regimes depending on the ratio between the electron wavelength and the interatomic distance： atomic regime, screened atomic regime, and quasi-molecular regime. This classification agrees qualitatively with the conclusion of full quantum-mechanical approach [Yonger S Met al 1988 Phys. Rev. Lett. 61 962].
基金supported by the National Key Research and Development Program of China (Grant No. 2019YFA0307704)the National Natural Science Foundation of China (Grant Nos. 11974425 and 11974426)。
文摘Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually,thereby greatly enhancing the scope of Stark decelerated molecules.For comparison,we detect decelerated and trapped ammonia molecules using two different schemes:(ⅰ) nonresonant multiphoton ionization using intense femtosecond(fs) pulses in the near infrared,and(ⅱ) resonance-enhanced multiphoton ionization using nanosecond(ns) pulses from a tunable UV laser.The observed number of ions per shot for both schemes is similar.The fs laser detection scheme suffers from an increased background,which can be effectively eliminated by subsequent mass and velocity selection.To determine the detection volume of the ns laser detection scheme,we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below~100 μm.It is concluded that the detection volume for the ns laser detection scheme is 1.5-2 times larger than that of the fs laser detection scheme.
基金supported by the National Key Research and Development Program of China(No.2019YFA0307704)the National Natural Science Foundation of China(Nos.11974425 and 11974426)the National Safety Academic Fund(No.U1830206)。
文摘We experimentally demonstrate third-harmonic generation(THG)in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic(SH).The mechanism of THG has been investigated,and it demonstrates that a third-order nonlinear process dominates at low pump intensity.Asymmetric third-harmonic(TH)spectra are observed at different time delays in two color fields,which are attributed to the process of the four-wave mixing(FWM)of the broad spectrum of pump pulses.A joint measurement on the terahertz(THz)and the TH is performed.It reveals that the optimized phase for the THG jumps from 0 to 0.5πas the pump intensity increases,which is different from the THz being a constant,and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.
