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High-order harmonic generation in atomic gases is important for several applications in ultrafast strong-field physics, ranging from attosecond pulse generation to ultrafast spectroscopy and imaging of different forms of matter. In the case of the generation with focused short Gaussian pulses, recent theoretical studies indicate that the conversion efficiency depends on the spatial phase distribution of the driving laser pulse which scales with the Porras factor. Using theoretical analysis and the results of numerical simulations, we find that for positive Porras factors the contribution of the Gouy phase to phase matching can be balanced and the conversion efficiency can be significantly enhanced as compared to a standard laser setup. Specifically, our results indicate that for a Porras factor of g0 = 1.2, the conversion efficiency as well as the cutoff of the harmonic spectra can be optimized while the harmonic lines remain narrow, which may be interesting for spectroscopic applications. 

We present an alternative way of calculating the Keldysh amplitude, i.e., the length-gauge form of the ionization amplitude in the strong-field approximation. The amplitude is evaluated exactly by expanding it in Fourier components and partial waves. Comparisons of the semianalytic model predictions with results of ab initio numerical simulations of the time-dependent Schrödinger equation for the interaction of electrons in short-range potentials with intense laser light yield excellent agreement, for wavelengths from the single photon to the multiphoton to the tunneling regime. Specifically, for ionization from initial states with higher angular momentum quantum number, e.g., p states, a significant improvement over predictions based on the popular saddle-point approximation is found. Furthermore, the current model rate allows for interpretation of the strong-field ionization process in terms of multiphoton absorption pathways and angular momentum selection rules.