1：Experimental Design and Method Selection:

The experiment uses a Ti:Sapphire laser system to generate high harmonics in a differentially pumped gas cell. The design includes a gas jet enclosed in a cell with differential pumping to control pressure. Theoretical models (segment source and distributed source models) are employed to analyze phase matching and absorption effects.

2：Sample Selection and Data Sources:

Nitrogen gas (N2) and atmospheric air are used as the gaseous media. The selection is based on their relevance to HHG studies, with air containing about 78% N

3：List of Experimental Equipment and Materials:

Ti:Sapphire laser system (800 nm, 50 fs pulses, up to 1 mJ pulse energy, 1 kHz repetition rate), 40 cm focal length lens, gas jet made from a 1 mm diameter nickel tube, differentially pumped cell with input and output holes, roughing pump (Oerlikon SC 30D), XUV spectrometer (McPherson 248/310 G) with grating (133.6 groves/mm), micro-channel plate (MCP), phosphor screen, charge-coupled device (CCD) camera.

4：6 groves/mm), micro-channel plate (MCP), phosphor screen, charge-coupled device (CCD) camera. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: The laser beam is focused into the gas jet using the lens. Harmonics are generated in the interaction region (R2). The gas pressure is varied, and HHG spectra are measured using the XUV spectrometer. Differential pumping is enabled to achieve high pressures and reduce reabsorption.

5：2). The gas pressure is varied, and HHG spectra are measured using the XUV spectrometer. Differential pumping is enabled to achieve high pressures and reduce reabsorption. Data Analysis Methods:

5. Data Analysis Methods: HHG yields are measured as a function of pressure. Theoretical calculations using one-dimensional models (Eqs. 1 and 3) are performed to compare with experimental data, involving coherence length, absorption length, and phase mismatch calculations.