We report the detection of near-IR H _ { 2 } emission from the low-ionization structures ( knots ) in two planetary nebulae .
The deepest ever high-angular resolution H _ { 2 } 1-0 S ( 1 ) at 2.122 µm , H _ { 2 } 2-1 S ( 1 ) at 2.248 µm and Br \gamma images of K 4-47 and NGC 7662 , obtained using the Near InfraRed Imager and Spectrometer ( NIRI ) at Gemini-North , are analyzed here .
K 4-47 reveals a remarkable highly collimated bipolar structure not only in the optical but also in the molecular hydrogen emission .
The H _ { 2 } emission emanates from the walls of the bipolar outflows and also from the pair of knots at the tip of the outflows .
The H _ { 2 } 1-0 S ( 1 ) /2-1 S ( 1 ) line ratio ranges from \sim 7 to \sim 10 suggesting the presence of shock interactions .
Our findings can be explained by the interaction of a jet/bullet ejected from the central star with the surrounding asymptotic giant branch material .
The strongest H _ { 2 } line , v=1-0 S ( 1 ) , is also detected in several low-ionization knots located at the periphery of the elliptical planetary nebula NGC 7662 , but only four of these knots are detected in the H _ { 2 } v=2-1 S ( 1 ) line .
These four knots exhibit an H _ { 2 } line ratio between 2 and 3.5 , which suggests that the emission is caused by the UV ionizing flux of the central star .
Our data confirms the presence of H _ { 2 } gas in both fast- and slow-moving low-ionization knots , which has only been confirmed before in the nearby Helix nebula and Hu 1-2 .
Overall , the low-ionization structures of planetary nebulae are found to share similar traits to photodissociation regions .