| Summary: | Photoionization dynamics of bounded electrons in the ground state, the first and second excited states of a hydrogen atom, triggered by ultrashort near-infrared laser pulses, have been investigated in a transition regime (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>γ</mi><mo>∼</mo><mn>1</mn></mrow></semantics></math></inline-formula>) that offers both multiphoton and tunneling features. Significant differences in spectral characteristics are found between the three low-energy states. The H(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>2</mn><mi>s</mi></mrow></semantics></math></inline-formula>) ionization probability is larger than the H(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>2</mn><mi>p</mi></mrow></semantics></math></inline-formula>) value with a special oscillating structure, but both are much greater than the ground state H(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>1</mn><mi>s</mi></mrow></semantics></math></inline-formula>) in a wide range of laser intensities. By comparing the momentum spectrum and angular distributions of low-energy photoelectrons released from these degenerate states, we find the H(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>2</mn><mi>p</mi></mrow></semantics></math></inline-formula>) state shows a stronger long-range Coulomb attraction force than the H(<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>2</mn><mi>s</mi></mrow></semantics></math></inline-formula>) state on account of the difference in the initial electron wave packet. Furthermore, analysis of the photoelectron momentum distributions sheds light on both the first and second excited states with a symmetrical intercycle interference structure in a multicycle field but an intracycle interference of an asymmetric left-handed or right-handed rotating spectrum in a few-cycle field. By analyzing photoelectron spectroscopy, we identify the parity characteristics of photoelectrons in different energy intervals and their corresponding above-threshold single-photon ionization (ATSI) or above-threshold double-photon ionization (ATDI) processes. We finally present the momentum distributions of the electrons ionized by laser pulses with different profiles and find the carrier-envelope phase (CEP) is a strong factor in deciding the rotating structure of the emission spectrum, which provides a new method to distinguish the CEP of few-cycle pulses.
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